Users with duplicate usernames appear in the system

In some (rare) circumstances, it appears possible for there to be two identical users in the JIRA database (where userbase.username is the same). Restoring a database in this state will result in the following exception at the end when reindexing (the database is still useable):

2005-07-08 10:51:47,451 WARN [jira.issue.index.DefaultIndexManager] There was an exception whilst reindexing issue ABC-30java.lang.IllegalArgumentException: Passed List had more than one value.
java.lang.IllegalArgumentException: Passed List had more than one value.
at org.ofbiz.core.entity.EntityUtil.getOnly(EntityUtil.java:58)
at com.opensymphony.user.provider.ofbiz.OFBizAbstractProvider.findUser(OFBizAbstractProvider.java:159)
at com.opensymphony.user.provider.ofbiz.OFBizCredentialsProvider.handles(OFBizCredentialsProvider.java:114)
at com.opensymphony.user.UserManager.getProvider(UserManager.java:313)
at com.opensymphony.user.UserManager.getEntity(UserManager.java:299)
at com.opensymphony.user.UserManager.getUser(UserManager.java:193)
at com.atlassian.core.user.UserUtils.getUser(UserUtils.java:31)
at com.atlassian.jira.issue.index.AbstractDocument.indexUserGroups(AbstractDocument.java:34)
at com.atlassian.jira.issue.index.IssueDocument.getDocument(IssueDocument.java:47)
at com.atlassian.jira.issue.index.DefaultIndexManager.indexIssue(DefaultIndexManager.java:154)
at com.atlassian.jira.issue.index.DefaultIndexManager.indexIssuesAndComments(DefaultIndexManager.java:272)
at com.atlassian.jira.issue.index.DefaultIndexManager.reIndexIssues(DefaultIndexManager.java:222)
at com.atlassian.jira.issue.index.DefaultIndexManager.reIndexAll(DefaultIndexManager.java:135)
at com.atlassian.jira.issue.index.DefaultIndexManager.activate(DefaultIndexManager.java:104)
at com.atlassian.jira.action.admin.DataImport.reindex(DataImport.java:202)
at com.atlassian.jira.action.admin.DataImport.doExecute(DataImport.java:140)
at webwork.action.ActionSupport.execute(ActionSupport.java:153)

The duplicate record can be identified with SQL:

mysql> select * from userbase group by username having count(username) > 1;
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