Last Friday evening I upgraded from Leopard to Snow Leopard on my iMac ( 2007 ) and since I have twice received over 260 duplicate emails from my ISP which is ATT. I don't know what to do to correct this. Any suggestions?

Is this a POP3 or IMAP account?

Check your account settings. If this is a POP account, look at the Advanced tab and make sure you check "Remove copy from server".

Thank you for the reply. I have changed the Advanced setting from 1 week to 1 day. However I am still puzzled as to what changed since the former setting had never caused a problem. I also noticed that in your screen shot, you do not have the "Include when automatically checking for new messages" option checked. I do and I am wondering exactly what that refers to.

What changed was your program! There's no knowing just what code changes were made inside of Mail (and if any bugs were introduced, or how other behaviors have changed).

The POP account I used for an example is an account I do *not* want checked automatically, so that's not checked. You should have yours checked if you want Mail to check it automatically.