Corrupted User(s) - LDAP-Sync fails

We have the same error like described in this issue: https://jira.atlassian.com/browse/CONF-31772

The problem seems to occur to one specific user. That user can't be LDAP synchronised, can't login and you cannot open the User Detail information or the Group Detail information that user belongs got. We always get following exception:

org.springframework.dao.IncorrectResultSizeDataAccessException: query did not return a unique result: 2
at org.springframework.orm.hibernate.SessionFactoryUtils.convertHibernateAccessException(SessionFactoryUtils.java:590)
at org.springframework.orm.hibernate.HibernateAccessor.convertHibernateAccessException(HibernateAccessor.java:353)
at org.springframework.orm.hibernate.HibernateTemplate.execute(HibernateTemplate.java:375)
at org.springframework.orm.hibernate.HibernateTemplate.execute(HibernateTemplate.java:337)
at com.atlassian.crowd.embedded.hibernate2.HibernateUserDao.internalFindUser(HibernateUserDao.java:474)
at com.atlassian.crowd.embedded.hibernate2.HibernateUserDao.internalFindByName(HibernateUserDao.java:451)
at com.atlassian.crowd.embedded.hibernate2.HibernateUserDao.findByName(HibernateUserDao.java:431)
at com.atlassian.confluence.user.crowd.CachedCrowdUserDao$1.apply(CachedCrowdUserDao.java:76)
at com.atlassian.confluence.user.crowd.CachedCrowdUserDao$1.apply(CachedCrowdUserDao.java:71)
at com.atlassian.confluence.cache.option.OptionalReadThroughCache.get(OptionalReadThroughCache.java:33)
at com.atlassian.confluence.user.crowd.CachedCrowdUserDao.findUser(CachedCrowdUserDao.java:136)
at com.atlassian.confluence.user.crowd.CachedCrowdUserDao.findByName(CachedCrowdUserDao.java:158)
at sun.reflect.GeneratedMethodAccessor263.invoke(Unknown Source)
(...)

We are not able te remove the user from the system. When we exclude the user in den LDAP sync configuration, do a restart of the server, the user entry seems to disappear. But when we try to sync the user from LDAP again, the LDAP synchronisation fails with the above exception.

How can we get rid off this "corrupted" user entry? Does that entry reside in the database or in a local cache file? How did this corrupted user entry even happen?

Or: it is possible there are much more such users in our productive system with full LDAP directory. How to recognize those?