You mean 0.3.1 to 0.4.1?
Yes.
Are your users logging in with full email address?
No, users only need their login without the @domain
Does the duplicated usernames differ with letter case?
No, the only difference i noticed was the language, in some cases it was pt_BR at one ocurrence and pt_br on another.
Is your IMAP server case sensitive on login?
Yes it is.
Thanks!
List info: http://lists.roundcube.net/users/ BT/9b404e9e