Browser/Email :: Receiving Multiple Copies Of Some Emails?

Using Outlook 2016, Windows 10. In the last few days, I've started getting multiple copies of the same email. Not all emails, but most. Some, just two or three. Others, up to a dozen of the same email. I've got four accounts at two different servers (RoadRunner and GoDaddy) ... and it's happening to all four, which is why I think it's an Outlook problem.

Additional clue: Some of the emails I'm getting multiple copies of are from last month ... stuff that had been deleted many days ago.

First instinct is to look at my settings (both POP accounts), to see if things are set up correctly. But that makes no sense - up until two or three days ago, everything was working exactly the way it should. I haven't made any system changes, haven't installed new software ... can't think of anything that would've made this just start happening.