Windows Live Mail keeps restoring my deleted Gmail emails

When I start up Windows Live mail, it loads up any new emails that I have received, then it goes on to restore all of the old emails that I have deleted over the last six months from my Gmail account. I have deleted these old emails countless times, but they keep coming back! It has become a major nuisance. It seems to happen most often whenever the Gmail account is highlighted, meaning that I must've been in that account when Windows Live Mail was closed last time. (I hope this makes sense.)