Problem w/ Yahoo and POP3: Duplicate messages

I have a yahoo mail account I use as a central collection for all my other e-mail addresses, and usually access it via POP3. Starting last week I’ve noticed a weird problem. Every time I do a “send a receive,” I’ll get a fresh copy of new messages. This happens with and Entourage on OSX and Outlook on XP, so it’s not some funky OS/Client issue.

For example:

I get a mail message. I do (1) send/receive and get (1) copy of that mail message. After (4) syncs I’ll have (4) copies. It’s a pain in the ass. I’m inclined to say “fuck it” and just forward my yahoo mail to GMAIL, but I’ve got a lost of historical e-mail on Yahoo I’d like to reference via the web front end if I need to. It doesn’t look like I can both autoforward a message from Yahoo AND keep a copy on Yahoo.

Any suggestions?