From: Brian Tillman [MVP-Outlook] on
"The Rock" <TheRock(a)> wrote in message

> Furthermore I left Outlook 2010 open since 16-5-2010, carefully not touching
> the Send and Receive button. Now the duplicate mail problem seems to be
> gone.
> Initially Outlook did not get my new mail, now it does. May be Outlook did a
> overnight background consistency check on the pst.

Not likely.

> Just jet I pressed Send Receive All Folders, this ended rapidly without
> duplicating mail. So far so good.
> So I restarted Outlook, pressed Send/Receive all Folders and again Outlook
> starts duplicating all my mail. I cancelled the Send/Receive all Folders. Up
> to now I manually deleted 6404 duplicate mails....

As I said before, this is an indication that Outlook isn't closing properly
and is not writing its internal data properly so it's not remembering what has
been downloaded. If this were to happen to me, I'd start with a new mail
profile. See if something here helps:
Brian Tillman [MVP-Outlook]