I asked the following question over on the Office 2013 forum and the only response I got basically told me to "go ask this on the Outlook forum", so here goes:
"I too am experiencing rabid email duplication with Outlook 2013. I've found a way to avoid the problem, and also a way to fairly reliably CAUSE it. I'm posting in the hope that this will help Microsoft with information that will allow them to correct their buggy code.
My configuration:
LAN/Domain network running Server 2008 R2 and a 3rd party (not Microsoft) email server. (Code-Crafters Ability Mail Server).
Physical network is a mix of hard-wired Gigabit Cat6 and wireless (Ubiquiti Pico M2 HP).
A mix of Windows workstations, desktops and laptops running a mix of XP-Pro, Vista Ultimate, Win 7 Pro, and 8.1 Pro.
A mix of Outlook versions, the XP and Vista machines are mostly running Outlook 2003 (yeah, I know, it's ancient but it generally still works fine for these users and they hate change). The 7 and 8.1 users are all on Outlooks 2013 (installed as Office 2013 Professional Plus 2013).
All of the users access their email on the OEM Ability server via IMAP. All users have at least "a couple" of sub-folders defined in their account and all use at least "some" Outlook rules to route some email at both receive and send time into various folders. Some users have a lot of rules and folders, some have only a small handful. These users also have external IMAP access to their email on the server via their Android phones, generally using the "Type" (formerly Bluemail) Android email client.
Everything from the server down (including the email server and the domain controller) is mine and under my direct control.
What happens fairly consistently for a few users is this:
If they have Outlook 2013 running on more than one workstation at the same time, eventually it will go insane duplicating a handful of emails, both in the inbox and in one or more of their folders, at the rate of once every few seconds. They are also connected via their android phone when this happens. All clients (all the Outlook instances and their phones) go nuts trying to send 'new mail' notifications out at a very rapid rate. It's not possible to delete them fast enough to keep up, no matter how quick they are on the trigger. The emails in question ARE in fact duplicates - everything about them including the header information matches - and they are "real" (the duplicate copies are also visible through the web interface directly into the email server), so I'm sure the duplication is not just inside the Outlook view of the user's email folder(s).
Stopping all of the user's instances of Outlook stops the duplication. Stopping just the android client does not. If the user shuts down all his/her instances of Outlook and uses the webmail interface to delete all the duplicates (even the 'original' email) and then starts exactly ONE instance of Outlook 2013, things are fine. Starting a second instance of Outlook 2013 on another machine (for the same user) will eventually cause the duplication to start again. Sometimes immediately, sometimes not, but in my experience it has always started again within an hour of starting the second Outlook 2013 instance. When it starts, the duplicated mail is the same as before - even though the user previously deleted all copies of it, they 'magically' start re-appearing.
I'd really like for this to get fixed. Now, before anyone replies, if you're going to just write "you shouldn't run multiple instances of Outlook for the same account from different machines", don't. I don't want to hear it and the reasons that some of my users want to do this are none of your concern. If it's DOCUMENTED in official Microsoft documentation that this is not a supported action, then I want to see the reference myself. Otherwise, keep it to yourself - thanks.
Also, don't suggest that I delete and rebuild the user's email profiles and/or email accounts on the server. Been there, done that (multiple times), it doesn't solve the problem.
Dont' suggest that I ditch my existing email server and convert to something else (like Exchange). I happen to like my existing email server (and so do my users). I've already worked this problem with their support structure fairly extensively and we're both satisfied that the problem is not being caused by their server code.
Finally, don't tell me to run scanpst. Also been there, done that (multiple times) and that too does not fix the problem, so either the problem is somewhere other than in a corrupted pst/ost file, or scanpst is too dumb to find and correct whatever corruption might exist.
Having said all that, any insights into what's going on?"
Another symptom, which may or may not be related that I'm seeing on one of the problem machines is that Outlook "sometimes" does not show recent email on the user's account. The user can access his email account via direct web access into the email server and is able to see, sometimes, several days worth of email traffic that does not appear in Outlook (again, using an IMAP connection). Trying various combinations of "send/receive" operations, "folder unsubscribe/subscribe" operations, even exiting and restarting Outlook all seem to complete with no errors, but also without retrieving the missing email from the server. Sometimes "time goes by" and the user can restart Outlook and the email will appear, but I've not yet found a way to make it happen.
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