Entire email going back to 2013 downloaded

[#1]

This morning I accessed my Plusnet email via Windows Live Mail as I do every day and have done for many years.
Over 1000 emails going back to January 10th 2013 were re-downloaded!
I now have duplicate emails (marked as unread) which I have to manually delete. This is going to take a long time.
I've been holding on the phone for over half an hour so far to try to talk to Tech Support, but no success so far!
Does anyone have any idea what has caused this and how I can delete the 1000+ duplicates?

[#2]

It's highly unlikely to be an issue at our side (unless others have noticed the same?), so it might be an idea to hang-up
I'm guessing you retrieve your email using POP3 and leave a copy of your messages on the server? If so, there's a POP3UIDL file stored locally on your machine that keeps track of what has/hasn't been downloaded. If that's become corrupted, then it may have fooled Window Live Mail into thinking all of the messages on the server are new and therefore need downloading.

[#3]

No point trying to speak to Plusnet as the issue is at your end.
You must have Live Mail using POP3 with it set to leave the emails on the server. It's then forgotten which emails it has already downloaded.
I suggest you log in to your mailbox using Webmail and make sure that emails are actually being deleted.
Edit: Bob posted while I was typing!

[#4]

Thanks guys!
I've just come off the phone and the result is basically what you've both just said.
Probably some weird kind of corruption in WLM that makes it think it hasn't downloaded emails that were.
I've now changed my WLM setting to 'delete emails from server after 10 days' . . . . so if this ever happens again, at least I'll only get 10 days worth of duplicates instead of 2 years worth!
Thanks for your help.