Yeah it seems you have users with duplicated mailboxes, you should only ever have 1 per user, if this is occurring you have a major cleanup task at hand and you'll want to look at that ASAP as you probably could be having messages ending up in thise o365 mailboxes. I had it happen to a few random users during my Hybrid migration where 365 would provision a mailbox for a user assigned a license, it was a pain, it's been a long time but if I remember correctly this article should cover what happend if that is indeed the case: https://blogs.technet.microsoft.com/exovoice/2016/11/07/how-to-fix-office365-user-provisioning-issue...
