[Dovecot] Multiple copies of a message

I have a problem and I don't know which part of the mail chain could be causing it, although dovecot does seem to be the most logical source.

I get multiple copies of messages to my Inbox. I have just deleted 24 copies of a newsletter from Computer Shopper. The worst example I have seen of this had more than 90 copies. This doesn't happen to any subfolder of Inbox, just Inbox itself.

The problem started about 10-14 days ago, on the old mail server (running Mandriva 2007.1). The new server runs CentOS and exhibits the same problem.

I'd be interested in any comments, as my efforts to track it down have got nowhere.

Anne