T184680: Update duplicate handling in reading lists API requires an index change to to wikishared.reading_list on x1. It is a single shared table used by all wikis, the code handling it is not exposed to traffic yet but will start getting beta tester traffic next week.
The required change is
ALTER IGNORE TABLE reading_list MODIFY rl_name VARCHAR(255) BINARY, DROP INDEX rl_user_deleted_name_id, ADD UNIQUE INDEX rl_user_deleted_name (rl_user_id, rl_deleted, rl_name), DROP INDEX rl_user_name_id, ADD UNIQUE INDEX rl_user_name (rl_user_id, rl_name);
This change will break existing code very slightly: attempts to create multiple lists with the same name will result in a duplicate key DB error; the corresponding patch will change that to a soft error. Also, creation of a new list with the same name as the deleted list will result in a duplicate key error; the patch will handle that situation correctly. ReadingLists is not-even-beta and not exposed to any real users so that's not considered a problem. Can probably be improved though.