Multiple Copies of Mail Problem

Every now and again (only had Mac two months) my mail (which works well most of the time) will receive the same email message multiple times, over and over, all day long.
Nothing has changed on my only account set up. Ports are correct, etc., etc.
This is driving me nuts and have never had such a problem with PC/Outlook.

Is Thunderbird or any other email app better than this Apple provided app?

Thanks for any help offered.