diff --git a/services/identity/src/grpc_services/authenticated.rs b/services/identity/src/grpc_services/authenticated.rs
index 29a48511b..f937f65c9 100644
--- a/services/identity/src/grpc_services/authenticated.rs
+++ b/services/identity/src/grpc_services/authenticated.rs
@@ -1,554 +1,575 @@
 use std::collections::HashMap;
 
 use crate::config::CONFIG;
 use crate::database::{DeviceListRow, DeviceListUpdate};
 use crate::{
   client_service::{handle_db_error, UpdateState, WorkflowInProgress},
   constants::request_metadata,
   database::DatabaseClient,
   ddb_utils::DateTimeExt,
   grpc_services::shared::get_value,
 };
 use chrono::{DateTime, Utc};
 use comm_opaque2::grpc::protocol_error_to_grpc_status;
 use tonic::{Request, Response, Status};
 use tracing::{debug, error, warn};
 
 use super::protos::auth::{
   identity, identity_client_service_server::IdentityClientService,
   GetDeviceListRequest, GetDeviceListResponse, Identity, InboundKeyInfo,
   InboundKeysForUserRequest, InboundKeysForUserResponse, KeyserverKeysResponse,
   LinkFarcasterAccountRequest, OutboundKeyInfo, OutboundKeysForUserRequest,
   OutboundKeysForUserResponse, RefreshUserPrekeysRequest,
   UpdateDeviceListRequest, UpdateUserPasswordFinishRequest,
   UpdateUserPasswordStartRequest, UpdateUserPasswordStartResponse,
   UploadOneTimeKeysRequest,
 };
 use super::protos::unauth::Empty;
 
 #[derive(derive_more::Constructor)]
 pub struct AuthenticatedService {
   db_client: DatabaseClient,
 }
 
 fn get_auth_info(req: &Request<()>) -> Option<(String, String, String)> {
   debug!("Retrieving auth info for request: {:?}", req);
 
   let user_id = get_value(req, request_metadata::USER_ID)?;
   let device_id = get_value(req, request_metadata::DEVICE_ID)?;
   let access_token = get_value(req, request_metadata::ACCESS_TOKEN)?;
 
   Some((user_id, device_id, access_token))
 }
 
 pub fn auth_interceptor(
   req: Request<()>,
   db_client: &DatabaseClient,
 ) -> Result<Request<()>, Status> {
   debug!("Intercepting request to check auth info: {:?}", req);
 
   let (user_id, device_id, access_token) = get_auth_info(&req)
     .ok_or_else(|| Status::unauthenticated("Missing credentials"))?;
 
   let handle = tokio::runtime::Handle::current();
   let new_db_client = db_client.clone();
 
   // This function cannot be `async`, yet must call the async db call
   // Force tokio to resolve future in current thread without an explicit .await
   let valid_token = tokio::task::block_in_place(move || {
     handle
       .block_on(new_db_client.verify_access_token(
         user_id,
         device_id,
         access_token,
       ))
       .map_err(handle_db_error)
   })?;
 
   if !valid_token {
     return Err(Status::aborted("Bad Credentials"));
   }
 
   Ok(req)
 }
 
 pub fn get_user_and_device_id<T>(
   request: &Request<T>,
 ) -> Result<(String, String), Status> {
   let user_id = get_value(request, request_metadata::USER_ID)
     .ok_or_else(|| Status::unauthenticated("Missing user_id field"))?;
   let device_id = get_value(request, request_metadata::DEVICE_ID)
     .ok_or_else(|| Status::unauthenticated("Missing device_id field"))?;
 
   Ok((user_id, device_id))
 }
 
 #[tonic::async_trait]
 impl IdentityClientService for AuthenticatedService {
   async fn refresh_user_prekeys(
     &self,
     request: Request<RefreshUserPrekeysRequest>,
   ) -> Result<Response<Empty>, Status> {
     let (user_id, device_id) = get_user_and_device_id(&request)?;
     let message = request.into_inner();
 
     debug!("Refreshing prekeys for user: {}", user_id);
 
     let content_keys = message
       .new_content_prekeys
       .ok_or_else(|| Status::invalid_argument("Missing content keys"))?;
     let notif_keys = message
       .new_notif_prekeys
       .ok_or_else(|| Status::invalid_argument("Missing notification keys"))?;
 
     self
       .db_client
       .update_device_prekeys(
         user_id,
         device_id,
         content_keys.into(),
         notif_keys.into(),
       )
       .await
       .map_err(handle_db_error)?;
 
     let response = Response::new(Empty {});
     Ok(response)
   }
 
   async fn get_outbound_keys_for_user(
     &self,
     request: tonic::Request<OutboundKeysForUserRequest>,
   ) -> Result<tonic::Response<OutboundKeysForUserResponse>, tonic::Status> {
     let message = request.into_inner();
     let user_id = &message.user_id;
 
     let devices_map = self
       .db_client
       .get_keys_for_user(user_id, true)
       .await
       .map_err(handle_db_error)?
       .ok_or_else(|| tonic::Status::not_found("user not found"))?;
 
     let transformed_devices = devices_map
       .into_iter()
       .map(|(key, device_info)| (key, OutboundKeyInfo::from(device_info)))
       .collect::<HashMap<_, _>>();
 
     Ok(tonic::Response::new(OutboundKeysForUserResponse {
       devices: transformed_devices,
     }))
   }
 
   async fn get_inbound_keys_for_user(
     &self,
     request: tonic::Request<InboundKeysForUserRequest>,
   ) -> Result<tonic::Response<InboundKeysForUserResponse>, tonic::Status> {
     use identity::IdentityInfo;
 
     let message = request.into_inner();
     let user_id = &message.user_id;
 
     let devices_map = self
       .db_client
       .get_keys_for_user(user_id, false)
       .await
       .map_err(handle_db_error)?
       .ok_or_else(|| tonic::Status::not_found("user not found"))?;
 
     let transformed_devices = devices_map
       .into_iter()
       .map(|(key, device_info)| (key, InboundKeyInfo::from(device_info)))
       .collect::<HashMap<_, _>>();
 
     let identifier = self
       .db_client
       .get_user_identifier(user_id)
       .await
       .map_err(handle_db_error)?;
 
     let identity_info = IdentityInfo::try_from(identifier)?;
 
     Ok(tonic::Response::new(InboundKeysForUserResponse {
       devices: transformed_devices,
       identity: Some(Identity {
         identity_info: Some(identity_info),
       }),
     }))
   }
 
   async fn get_keyserver_keys(
     &self,
     request: Request<OutboundKeysForUserRequest>,
   ) -> Result<Response<KeyserverKeysResponse>, Status> {
     use identity::IdentityInfo;
 
     let message = request.into_inner();
 
     let keyserver_info = self
       .db_client
       .get_keyserver_keys_for_user(&message.user_id)
       .await
       .map_err(handle_db_error)?
       .map(OutboundKeyInfo::from);
 
     let identifier = self
       .db_client
       .get_user_identifier(&message.user_id)
       .await
       .map_err(handle_db_error)?;
 
     let identity_info = IdentityInfo::try_from(identifier)?;
 
     let identity = Some(Identity {
       identity_info: Some(identity_info),
     });
 
     let response = Response::new(KeyserverKeysResponse {
       keyserver_info,
       identity,
     });
 
     return Ok(response);
   }
 
   async fn upload_one_time_keys(
     &self,
     request: tonic::Request<UploadOneTimeKeysRequest>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     let (user_id, device_id) = get_user_and_device_id(&request)?;
     let message = request.into_inner();
 
     debug!("Attempting to update one time keys for user: {}", user_id);
     self
       .db_client
       .append_one_time_prekeys(
         device_id,
         message.content_one_time_prekeys,
         message.notif_one_time_prekeys,
       )
       .await
       .map_err(handle_db_error)?;
 
     Ok(tonic::Response::new(Empty {}))
   }
 
   async fn update_user_password_start(
     &self,
     request: tonic::Request<UpdateUserPasswordStartRequest>,
   ) -> Result<tonic::Response<UpdateUserPasswordStartResponse>, tonic::Status>
   {
     let (user_id, _) = get_user_and_device_id(&request)?;
     let message = request.into_inner();
 
     let server_registration = comm_opaque2::server::Registration::new();
     let server_message = server_registration
       .start(
         &CONFIG.server_setup,
         &message.opaque_registration_request,
         user_id.as_bytes(),
       )
       .map_err(protocol_error_to_grpc_status)?;
 
     let update_state = UpdateState { user_id };
     let session_id = self
       .db_client
       .insert_workflow(WorkflowInProgress::Update(update_state))
       .await
       .map_err(handle_db_error)?;
 
     let response = UpdateUserPasswordStartResponse {
       session_id,
       opaque_registration_response: server_message,
     };
     Ok(Response::new(response))
   }
 
   async fn update_user_password_finish(
     &self,
     request: tonic::Request<UpdateUserPasswordFinishRequest>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     let message = request.into_inner();
 
     let Some(WorkflowInProgress::Update(state)) = self
       .db_client
       .get_workflow(message.session_id)
       .await
       .map_err(handle_db_error)?
     else {
       return Err(tonic::Status::not_found("session not found"));
     };
 
     let server_registration = comm_opaque2::server::Registration::new();
     let password_file = server_registration
       .finish(&message.opaque_registration_upload)
       .map_err(protocol_error_to_grpc_status)?;
 
     self
       .db_client
       .update_user_password(state.user_id, password_file)
       .await
       .map_err(handle_db_error)?;
 
     let response = Empty {};
     Ok(Response::new(response))
   }
 
   async fn log_out_user(
     &self,
     request: tonic::Request<Empty>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     let (user_id, device_id) = get_user_and_device_id(&request)?;
 
     self
       .db_client
       .remove_device(&user_id, &device_id)
       .await
       .map_err(handle_db_error)?;
 
     self
       .db_client
       .delete_access_token_data(user_id, device_id)
       .await
       .map_err(handle_db_error)?;
 
     let response = Empty {};
     Ok(Response::new(response))
   }
 
   async fn delete_user(
     &self,
     request: tonic::Request<Empty>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     let (user_id, _) = get_user_and_device_id(&request)?;
 
     self
       .db_client
       .delete_user(user_id)
       .await
       .map_err(handle_db_error)?;
 
     let response = Empty {};
     Ok(Response::new(response))
   }
 
   async fn get_device_list_for_user(
     &self,
     request: tonic::Request<GetDeviceListRequest>,
   ) -> Result<tonic::Response<GetDeviceListResponse>, tonic::Status> {
     let GetDeviceListRequest {
       user_id,
       since_timestamp,
     } = request.into_inner();
 
     let since = since_timestamp
       .map(|timestamp| {
         DateTime::<Utc>::from_utc_timestamp_millis(timestamp)
           .ok_or_else(|| tonic::Status::invalid_argument("Invalid timestamp"))
       })
       .transpose()?;
 
     let mut db_result = self
       .db_client
       .get_device_list_history(user_id, since)
       .await
       .map_err(handle_db_error)?;
 
     // these should be sorted already, but just in case
     db_result.sort_by_key(|list| list.timestamp);
 
     let device_list_updates: Vec<SignedDeviceList> = db_result
       .into_iter()
       .map(RawDeviceList::from)
       .map(SignedDeviceList::try_from_raw)
       .collect::<Result<Vec<_>, _>>()?;
 
     let stringified_updates = device_list_updates
       .iter()
       .map(serde_json::to_string)
       .collect::<Result<Vec<_>, _>>()
       .map_err(|err| {
         error!("Failed to serialize device list updates: {}", err);
         tonic::Status::failed_precondition("unexpected error")
       })?;
 
     Ok(Response::new(GetDeviceListResponse {
       device_list_updates: stringified_updates,
     }))
   }
 
   async fn update_device_list(
     &self,
     request: tonic::Request<UpdateDeviceListRequest>,
   ) -> Result<Response<Empty>, tonic::Status> {
     let (user_id, _device_id) = get_user_and_device_id(&request)?;
     // TODO: when we stop doing "primary device rotation" (migration procedure)
     // we should verify if this RPC is called by primary device only
 
     let new_list = SignedDeviceList::try_from(request.into_inner())?;
     let update = DeviceListUpdate::try_from(new_list)?;
     self
       .db_client
       .apply_devicelist_update(&user_id, update)
       .await
       .map_err(handle_db_error)?;
 
     Ok(Response::new(Empty {}))
   }
 
   async fn link_farcaster_account(
     &self,
     request: tonic::Request<LinkFarcasterAccountRequest>,
   ) -> Result<Response<Empty>, tonic::Status> {
     let (user_id, _) = get_user_and_device_id(&request)?;
     let message = request.into_inner();
 
+    let mut get_farcaster_users_response = self
+      .db_client
+      .get_farcaster_users(vec![message.farcaster_id.clone()])
+      .await
+      .map_err(handle_db_error)?;
+
+    if get_farcaster_users_response.len() > 1 {
+      error!("multiple users associated with the same Farcaster ID");
+      return Err(Status::failed_precondition("cannot link Farcaster ID"));
+    }
+
+    if let Some(u) = get_farcaster_users_response.pop() {
+      if u.0.user_id == user_id {
+        return Ok(Response::new(Empty {}));
+      } else {
+        return Err(Status::already_exists(
+          "farcaster ID already associated with different user",
+        ));
+      }
+    }
+
     self
       .db_client
       .add_farcaster_id(user_id, message.farcaster_id)
       .await
       .map_err(handle_db_error)?;
 
     let response = Empty {};
     Ok(Response::new(response))
   }
 }
 
 // raw device list that can be serialized to JSON (and then signed in the future)
 #[derive(serde::Serialize, serde::Deserialize)]
 struct RawDeviceList {
   devices: Vec<String>,
   timestamp: i64,
 }
 
 impl From<DeviceListRow> for RawDeviceList {
   fn from(row: DeviceListRow) -> Self {
     Self {
       devices: row.device_ids,
       timestamp: row.timestamp.timestamp_millis(),
     }
   }
 }
 
 #[derive(serde::Serialize, serde::Deserialize)]
 #[serde(rename_all = "camelCase")]
 struct SignedDeviceList {
   /// JSON-stringified [`RawDeviceList`]
   raw_device_list: String,
 }
 
 impl SignedDeviceList {
   /// Serialize (and sign in the future) a [`RawDeviceList`]
   fn try_from_raw(raw: RawDeviceList) -> Result<Self, tonic::Status> {
     let stringified_list = serde_json::to_string(&raw).map_err(|err| {
       error!("Failed to serialize raw device list: {}", err);
       tonic::Status::failed_precondition("unexpected error")
     })?;
 
     Ok(Self {
       raw_device_list: stringified_list,
     })
   }
 
   fn as_raw(&self) -> Result<RawDeviceList, tonic::Status> {
     // The device list payload is sent as an escaped JSON payload.
     // Escaped double quotes need to be trimmed before attempting to deserialize
     serde_json::from_str(&self.raw_device_list.replace(r#"\""#, r#"""#))
       .map_err(|err| {
         warn!("Failed to deserialize raw device list: {}", err);
         tonic::Status::invalid_argument("invalid device list payload")
       })
   }
 }
 
 impl TryFrom<UpdateDeviceListRequest> for SignedDeviceList {
   type Error = tonic::Status;
   fn try_from(request: UpdateDeviceListRequest) -> Result<Self, Self::Error> {
     serde_json::from_str(&request.new_device_list).map_err(|err| {
       warn!("Failed to deserialize device list update: {}", err);
       tonic::Status::invalid_argument("invalid device list payload")
     })
   }
 }
 
 impl TryFrom<SignedDeviceList> for DeviceListUpdate {
   type Error = tonic::Status;
   fn try_from(signed_list: SignedDeviceList) -> Result<Self, Self::Error> {
     let RawDeviceList {
       devices,
       timestamp: raw_timestamp,
     } = signed_list.as_raw()?;
     let timestamp = DateTime::<Utc>::from_utc_timestamp_millis(raw_timestamp)
       .ok_or_else(|| {
       error!("Failed to parse RawDeviceList timestamp!");
       tonic::Status::invalid_argument("invalid timestamp")
     })?;
     Ok(DeviceListUpdate::new(devices, timestamp))
   }
 }
 
 #[cfg(test)]
 mod tests {
   use super::*;
 
   #[test]
   fn serialize_device_list_updates() {
     let raw_updates = vec![
       RawDeviceList {
         devices: vec!["device1".into()],
         timestamp: 111111111,
       },
       RawDeviceList {
         devices: vec!["device1".into(), "device2".into()],
         timestamp: 222222222,
       },
     ];
 
     let expected_raw_list1 = r#"{"devices":["device1"],"timestamp":111111111}"#;
     let expected_raw_list2 =
       r#"{"devices":["device1","device2"],"timestamp":222222222}"#;
 
     let signed_updates = raw_updates
       .into_iter()
       .map(SignedDeviceList::try_from_raw)
       .collect::<Result<Vec<_>, _>>()
       .expect("signing device list updates failed");
 
     assert_eq!(signed_updates[0].raw_device_list, expected_raw_list1);
     assert_eq!(signed_updates[1].raw_device_list, expected_raw_list2);
 
     let stringified_updates = signed_updates
       .iter()
       .map(serde_json::to_string)
       .collect::<Result<Vec<_>, _>>()
       .expect("serialize signed device lists failed");
 
     let expected_stringified_list1 = r#"{"rawDeviceList":"{\"devices\":[\"device1\"],\"timestamp\":111111111}"}"#;
     let expected_stringified_list2 = r#"{"rawDeviceList":"{\"devices\":[\"device1\",\"device2\"],\"timestamp\":222222222}"}"#;
 
     assert_eq!(stringified_updates[0], expected_stringified_list1);
     assert_eq!(stringified_updates[1], expected_stringified_list2);
   }
 
   #[test]
   fn deserialize_device_list_update() {
     let raw_payload = r#"{"rawDeviceList":"{\"devices\":[\"device1\",\"device2\"],\"timestamp\":123456789}"}"#;
     let request = UpdateDeviceListRequest {
       new_device_list: raw_payload.to_string(),
     };
 
     let signed_list = SignedDeviceList::try_from(request)
       .expect("Failed to parse SignedDeviceList");
     let update = DeviceListUpdate::try_from(signed_list)
       .expect("Failed to parse DeviceListUpdate from signed list");
 
     let expected_timestamp =
       DateTime::<Utc>::from_utc_timestamp_millis(123456789).unwrap();
 
     assert_eq!(update.timestamp, expected_timestamp);
     assert_eq!(
       update.devices,
       vec!["device1".to_string(), "device2".to_string()]
     );
   }
 }