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
@@ -125,6 +125,10 @@
       return Err(tonic::Status::invalid_argument("username reserved"));
     }
 
+    if let Some(fid) = &message.farcaster_id {
+      self.check_farcaster_id_taken(fid).await?;
+    }
+
     let registration_state = construct_user_registration_info(
       &message,
       None,
@@ -539,6 +543,10 @@
       ));
     }
 
+    if let Some(fid) = &message.farcaster_id {
+      self.check_farcaster_id_taken(fid).await?;
+    }
+
     let flattened_device_key_upload =
       construct_flattened_device_key_upload(&message)?;
 
@@ -925,6 +933,24 @@
     Ok(())
   }
 
+  async fn check_farcaster_id_taken(
+    &self,
+    farcaster_id: &str,
+  ) -> Result<(), tonic::Status> {
+    let fid_already_registered = !self
+      .client
+      .get_farcaster_users(vec![farcaster_id.to_string()])
+      .await
+      .map_err(handle_db_error)?
+      .is_empty();
+    if fid_already_registered {
+      return Err(tonic::Status::already_exists(
+        "farcaster ID already associated with different user",
+      ));
+    }
+    Ok(())
+  }
+
   async fn verify_and_remove_nonce(
     &self,
     nonce: &str,