[horde] db integrity constraint violation while turba upgrade to 4.2.0

[Rene Zeipelt]

Hello,
while upgrading horde from 5.1.6 to 5.2 the turba upgrade reports the db 
migrating error:

[  INFO  ] Migrating DB up.
[  INFO  ] Current turba schema version: 9
Migrating to TurbaUpgradeCategoriesToTags (10)
== 10 TurbaUpgradeCategoriesToTags: migrating 
=================================
== 10 TurbaUpgradeCategoriesToTags: Migrating contact categories to 
tags. =====
-- select('SELECT object_uid, object_category, owner_id FROM turba_objects')
    -> 0.0006s
SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 
'ausername' for key 'rampage_users_user_name'

Is there a way to find the wrong data set manually?
The user did logins with "Ausername" and "ausername" (against a imap 
server).
Thanks for help.
Regards Rene Zeipelt
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