/errors/user_list_error.proto 

--- v23/errors/user_list_error.proto    2026-04-17 05:00:47.000000000 +0000
+++ v24/errors/user_list_error.proto    2026-04-17 05:00:55.000000000 +0000
@@ -144,6 +144,10 @@
     // One or more invalid country codes are added to Lookalike UserList.
     INVALID_COUNTRY_CODES = 45;

+    // Cannot create a Lookalike UserList which is a duplicate of an existing
+    // Lookalike.
+    DUPLICATE_LOOKALIKE = 46;
+
     // The partner audience source is not supported for the user list type.
     PARTNER_AUDIENCE_SOURCE_NOT_SUPPORTED_FOR_USER_LIST_TYPE = 47;