duplicate emails using OE with Hotmail

I look at my Hotmail account using Outlook Express. All incoming mail is duplicated. If there are 3 new messages my INBOX says there are 3 but I actually recieve 6! If I look at my Hotmail on the web I do not have this problem.
I also use OE to manage my email account with Yahoo. This mail is never duplicated. I’ve heard of this before but what is the cause & cure?