diff --git a/services/identity/src/client_service.rs b/services/identity/src/client_service.rs
--- a/services/identity/src/client_service.rs
+++ b/services/identity/src/client_service.rs
@@ -141,6 +141,12 @@
       message.username.clone(),
       message.farcaster_id.clone(),
     )?;
+    self
+      .check_device_id_taken(
+        &registration_state.flattened_device_key_upload,
+        None,
+      )
+      .await?;
     let server_registration = comm_opaque2::server::Registration::new();
     let server_message = server_registration
       .start(
@@ -197,6 +203,12 @@
       message.username.clone(),
       None,
     )?;
+    self
+      .check_device_id_taken(
+        &registration_state.flattened_device_key_upload,
+        None,
+      )
+      .await?;
     let server_registration = comm_opaque2::server::Registration::new();
     let server_message = server_registration
       .start(
@@ -322,6 +334,9 @@
 
     let flattened_device_key_upload =
       construct_flattened_device_key_upload(&message)?;
+    self
+      .check_device_id_taken(&flattened_device_key_upload, Some(&user_id))
+      .await?;
 
     let maybe_device_to_remove = self
       .get_keyserver_device_to_remove(
@@ -484,6 +499,10 @@
       return Err(tonic::Status::not_found("user not found"));
     };
 
+    self
+      .check_device_id_taken(&flattened_device_key_upload, Some(&user_id))
+      .await?;
+
     self
       .client
       .add_user_device(
@@ -573,6 +592,9 @@
 
     let flattened_device_key_upload =
       construct_flattened_device_key_upload(&message)?;
+    self
+      .check_device_id_taken(&flattened_device_key_upload, None)
+      .await?;
 
     let login_time = chrono::Utc::now();
 
@@ -658,6 +680,9 @@
 
     let flattened_device_key_upload =
       construct_flattened_device_key_upload(&message)?;
+    self
+      .check_device_id_taken(&flattened_device_key_upload, None)
+      .await?;
 
     let initial_device_list = message.get_and_verify_initial_device_list()?;
     let social_proof =
@@ -722,6 +747,10 @@
     let nonce = challenge_response.verify_and_get_nonce(&device_id)?;
     self.verify_and_remove_nonce(&nonce).await?;
 
+    self
+      .check_device_id_taken(&flattened_device_key_upload, Some(&user_id))
+      .await?;
+
     let user_identity = self
       .client
       .get_user_identity(&user_id)
@@ -1048,6 +1077,38 @@
     Ok(())
   }
 
+  async fn check_device_id_taken(
+    &self,
+    key_upload: &FlattenedDeviceKeyUpload,
+    requesting_user_id: Option<&str>,
+  ) -> Result<(), tonic::Status> {
+    let device_id = key_upload.device_id_key.as_str();
+    let Some(existing_device_user_id) = self
+      .client
+      .find_user_id_for_device(device_id)
+      .await
+      .map_err(handle_db_error)?
+    else {
+      // device ID doesn't exist
+      return Ok(());
+    };
+
+    // allow already-existing device ID for the same user
+    match requesting_user_id {
+      Some(user_id) if user_id == existing_device_user_id => {
+        debug!(
+          "Found already-existing device {} for user {}",
+          device_id, user_id
+        );
+        Ok(())
+      }
+      _ => {
+        warn!("Device ID already exists: {device_id}");
+        Err(tonic::Status::already_exists("device_already_exists"))
+      }
+    }
+  }
+
   async fn verify_and_remove_nonce(
     &self,
     nonce: &str,
diff --git a/services/identity/src/database/device_list.rs b/services/identity/src/database/device_list.rs
--- a/services/identity/src/database/device_list.rs
+++ b/services/identity/src/database/device_list.rs
@@ -746,11 +746,13 @@
     Ok(user_devices_keys)
   }
 
+  /// Find owner's user ID for given device ID. Useful for finding
+  /// devices table partition key.
   #[tracing::instrument(skip_all)]
-  pub async fn find_device_by_id(
+  pub async fn find_user_id_for_device(
     &self,
     device_id: &str,
-  ) -> Result<Option<DeviceRow>, Error> {
+  ) -> Result<Option<String>, Error> {
     let response = self
       .client
       .query()
@@ -785,15 +787,23 @@
       return Err(Error::IllegalState);
     }
 
-    let Some(user_id) = results
+    let user_id = results
       .pop()
       .map(|mut attrs| attrs.take_attr::<String>(devices_table::ATTR_USER_ID))
-      .transpose()?
-    else {
+      .transpose()?;
+
+    Ok(user_id)
+  }
+
+  #[tracing::instrument(skip_all)]
+  pub async fn find_device_by_id(
+    &self,
+    device_id: &str,
+  ) -> Result<Option<DeviceRow>, Error> {
+    let Some(user_id) = self.find_user_id_for_device(device_id).await? else {
       debug!("No device found with ID: {}", device_id);
       return Ok(None);
     };
-
     self.get_device_data(user_id, device_id).await
   }