[PM-7919] return exception if trying to overwrite keypair (#4052)

* return exception if trying to overwrite keypair

* add feature flag

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);
     }