"The fault, dear Brutus, is not in our stars, but in ourselves." -- Cassius, in Shakespeare's Julius Caesar
Cassius would have made an excellent tech support guru. When struck with a computer problem, we often assume the cause must lie with faulty software or hardware. Only later do we discover that the true cause is our own misunderstanding.
Case in point: I had an annoying symptom with Outlook 2011 for Mac. Periodically, when I received an email from friends, I would get two copies of the message in my Inbox. This duplication only seemed to happen when the message had multiple recipients. As such, my first thought was that my name had been included twice on the senders' cc-lists. However, careful checking of the lists determined such was not the case.
So what was going on? I could find no common thread that tied together the messages that were duplicated. Suspecting a bug in Outlook, I turned to Microsoft for help. To my pleasant surprise, a Microsoft webpage covered exactly this problem: Duplicate email messages appear in Outlook for Mac 2011.
I was skeptical that the page would contain the answer to my specific issue. But I gave it a try. Step 1 was "Check your rules" as found via the command in the Tools menu. That was as far as I needed to go. I went back to Outlook to take a careful look at the rules I had created for filtering my email. And there it was: a rule that essentially said "For any message with any To recipient that contains 'gmail.com,' copy the message to my Inbox."
I had created the rule a long time ago to deal with a problem with my Gmail address that was no longer relevant. I had forgotten the rule was even there.
As it turns out, my Gmail address is not my most commonly used address. My friends typically send me messages via another address. And those messages show up in my Inbox just fine. However, if the email contains multiple recipients, and one of those other recipients has a gmail address, my rule caused a second copy of the message to show up my Inbox! Duplication explained!
The solution was simple. I disabled the no-longer-needed rule. All is now well.
While the specifics on my situation are unlikely to directly apply to any duplication problems you have, the general advice is solid: Check your rules for possible confusion that may be the cause of your frustration.