To Many Copies

I just switched over to a new computer, set up my Yahoo mail in Vista's Windows Mail and when I went to download my email onto the new computer, all 4000+ emails over the last couple years downloaded. The next time I went to download email, all 4000+ emails want to download again. I now have 4 copies of some of the oldest emails. Is there a setting I need to change? Or does anyone know why it doesn't recognize them as already in my inbox so it feels the need to download them again?

thanks,

Chalrie