Outlook 2010 psts downloading duplicates - new issue

Question

Hi all,
I've seen so many posts regarding duplicate emails over the last few days but nothing that explains my exact problem and any solution to it so apologies if this is just re-wording an old problem
We've been using Outlook 2010 for the last 3 years that I've been with my current company and in that time I've deployed and redeployed most employees onto this version. 90% of them work off of pst files connecting to MDaemon messaging server.
In the past whenever I've upgraded someone to 2010, reinstalled 2010 onto their pc or upgraded their pc inc. 2010 I've always set their outlook up the same way:
Manually configure
POP3 account
Use an existing outlook pst file
advanced settings > leave a copy on the server > do not delete after n days
This has never caused me any issues. I've never had duplicates come down using this method and it has always just picked up where they last left off
In the last few weeks however we have used this exact same method on about 4 accounts and all of them have started downloading duplicates. The first few were only small (1000-2000 emails) and so we went through and removed the duplicates afterward. However
once we deleted these duplicates they then re-downloaded again. One account we had this delete/redownload/delete combo happen 4 times!
So far the only way I've found around it is to move all of their emails out of the IMAP inbox folder so that Outlook can't find anything to download. This does work but we've never had to do this before
The question is multipart:
1) Why is Outlook now downloading these duplicates when it never has before
2) Why does Outlook continue to download duplicates after a download/deletion has happened
3) How do we prevent these duplicates from happening in the pst file without removing emails from the IMAP folder
4) Where does Outlook store it's records of what's already downloaded
I've been told that what I've covered above is just the way Outlook works with pst files but I simply can't logically believe that. Fair enough on a new profile on a new machine I can understand that the setup may not know what has already been downloaded
but then there must be something somewhere that tells Outlook that it's already downloaded emails somehow, otherwise you would constantly be redownloading every email. And if this record is stored in the pst file itself then why would it download duplicates
if you use the same pst file elsewhere?
I'm struggling to understand and/or pinpoint why duplicates all of a sudden are now plaguing us. I've not had this issue in 3 yeares of using hte same method to set up accounts on new installs using existing pst files

Answer 1

Hi,
How about manually canceling the credential window, can Outlook still send/receive emails properly?
For this kind of credential issues, I would first try creating another Outlook profile and test if the issue persists.
Thanks,
Simon Wu
TechNet Community Support