IMAP on BlackBerry causing problems


We use POP3 on an outlook 2007 server, and we have a problem when we want to receive our mails on our blackberry’s:
randomly (sometimes several times a day), all the message still stored on the server are downloaded again both by the blackberry and by outlook 2007.

This is quite annoying and even if we configure Outlook as IMAP, it does the same (downloading and reindexing the IMAP folders).

I have searched the forums and saw someone had the same problem but no solution was mentionned, has anyone figured out how to fix this please?

Thanks in advance,