Summary:Extension talk:UserMerge/Database error A database query error has occurred. This may indicate a bug in the software.

Known problem.

Will be fixed soon, filed as Bug 39470: Can't merge blocked users: Duplicate entry for key 'ipb_address'.

See workaround in https://bugzilla.wikimedia.org/show_bug.cgi?id=39470 and https://www.mediawiki.org/w/index.php?title=Extension_talk:UserMerge&dir=prev&offset=20140124142442&limit=20&lqt_mustshow=40885#x.5BWORKAROUND.5D_Duplicate_entry_40885

Work-around: unblocking the user prior to using merge/delete solves the problem until the bug can be fixed.

Please, if your problem persists, then please add your comment to https://bugzilla.wikimedia.org/show_bug.cgi?id=39470, not here.