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 and Thread:Extension talk:UserMerge/Duplicate entry

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, not here.