diff --git a/src/Api/Auth/Controllers/AccountsController.cs b/src/Api/Auth/Controllers/AccountsController.cs
index 5f1910fb28..28135c4714 100644
--- a/src/Api/Auth/Controllers/AccountsController.cs
+++ b/src/Api/Auth/Controllers/AccountsController.cs
@@ -618,6 +618,14 @@ public class AccountsController : Controller
             throw new UnauthorizedAccessException();
         }
 
+        if (_featureService.IsEnabled(FeatureFlagKeys.ReturnErrorOnExistingKeypair))
+        {
+            if (!string.IsNullOrWhiteSpace(user.PrivateKey) || !string.IsNullOrWhiteSpace(user.PublicKey))
+            {
+                throw new BadRequestException("User has existing keypair");
+            }
+        }
+
         await _userService.SaveUserAsync(model.ToUser(user));
         return new KeysResponseModel(user);
     }
diff --git a/src/Core/Constants.cs b/src/Core/Constants.cs
index fddf4c6fe3..de86170ac2 100644
--- a/src/Core/Constants.cs
+++ b/src/Core/Constants.cs
@@ -109,6 +109,8 @@ public static class FeatureFlagKeys
     public const string Fido2VaultCredentials = "fido2-vault-credentials";
     public const string VaultOnboarding = "vault-onboarding";
     public const string BrowserFilelessImport = "browser-fileless-import";
+    public const string ReturnErrorOnExistingKeypair = "return-error-on-existing-keypair";
+
     /// <summary>
     /// Deprecated - never used, do not use. Will always default to false. Will be deleted as part of Flexible Collections cleanup
     /// </summary>