I have the same dupe email issue, as have others. Like you, I only see this issue on my work Outlook Exchange email account. None of my other email accounts suffer from this problem. I'm VZW, Pearl, on BIS.
There's a long thread here w/some attempted fixes but none have worked for me.
Here's the thread: Dreaded Duplicate Messages
Recently I believe I've noticed one variable that I didn't see before...My dupe problem may only be occurring when replying to emails when using RPC via HTTP access from home. I haven't confirmed it yet, but will check it out when I get back to work on Monday.
"RPC over HTTP" is a feature my company offers that allows employees to our work Outlook Exchange email accounts from home w/out having to log into our work VPN. That's a nice option, as I can still access my home printers and network resources using RPC over HTTP to read/reply to email, use my work calendar, etc.
If it turns out that my dupes are in fact limited to when I'm using RPC over HTTP, maybe I'll be one step closer to solving the issue...
If anyone here w/more technical knowledge has any thoughts about why RPC over HTTP might contribute to this issue, I'd love to hear your ideas/opinions!