Need help with Mail asap please

I need some help with the mail program on my MBP. For some reason this morning it has decided to keep getting the same emails over and over again and adding them as new. So now I am up to 3000 new emails even though I have already read and deleted them before. Every time I click for it to check for new mail it just downloads the old ones three and four times. How can I get it to stop downloading them again?