In case it wasn't Gmail, and is the Outlook/Exchange/IMAP duplicates problem, here's the relevant thread... Dreaded Duplicate Messages
And here's a link to a post from me in the thread...it's a somewhat kludgey work-around if you can't resolve the problem. Dreaded Duplicate Messages
I had gotten rid of my dupes from this issue for months, but then they came back when my firm made some change to our servers...dupes came back, so I've had to use my work-around to avoid the dupes since then.