Anyone have experience with iMail 12.5.2?

My brother in law works for company that is using iMail 12.5.2.  Most of their users are IMAP/SMTP and a couple of them are pop3/SMTP.  In any case, about 30% of them are complaining about duplicated emails in their inbox.  A common scenario is they start their day, check/download email, respond, delete, file email into folders, etc.  Then randomly, meaning there isn't a time of day or occurrence that will trigger this and sometimes it doesn't happen at all in a day, the random email will drop back into their inbox.

The duplicate email tends to be their most recent ones, but is not limited to it.  Any it isn't just that day's email.  So you may get duplicated email from today's batch, 10 days ago, (though the oldest seems to be 1 year).  Even stranger, it isn't contiguous, meaning the users get 20 from today, 50 from two weeks ago, and 10 from a month ago, with email in between that does not duplicate.  Has anyone heard of behavior like this?  The clients are Mac Outlook (Mavericks-pop3 and Yosemite-IMAP), Microsoft Live Mail (Windows 8.1-POP), Outlook 2003 (Windows 7-imap), and Outlook 2013 (Windows 8.1-imap).  There might be more, but so far, 5 people have complained of this.  My gut tells me the server's database is corrupted, since it is cross platform, cross protocol, and multiple users.

I am not the admin of the email server, so I just want to gather some information for the guy on places to look from people who have more experience with iMail.