Outlook sends duplicate emails

Question

Some users may have encounter the issue that Outlook sends duplicate emails to the recipients, this can be really frustrating as the
causes can be varied so that it’s really hard to troubleshoot . In this article, some most common causes will be introduced, which helps us troubleshoot this kind of
issue.
Send emails on Web first
Before we determine whether it is a server side issue or Outlook client side issue, please logon the webmail, or OWA for Exchange Accounts, send some test emails to have
the recipients to check again whether they receive duplicate emails. If the recipients don’t receive duplicate emails, we can tell it is probably an Outlook client issue.
Anti-Virus program
As some Anti-Virus programs may have been integrated with Outlook to keep the users from spams and viruses, it may also bring some issues sometimes. The program may also scan the outgoing emails and bring troubles, such as making the emails sent multiple
times.
Contact the program provider to know how to disable the integration with Outlook, and exit the Anti-Virus program temporarily, send test emails to see the result.
Similar to Anti-Virus program integration, it can be some add-ins, we can run Outlook in Safe Mode as well:
Run -> Type “outlook.exe /safe” in the blank box -> OK.
If the issue is gone when Outlook is running in Safe Mode, disabling the add-ins which may fix the
problem.
Send/Receive interval conflict
In Outlook we can set an interval according to which Outlook performs a Send/Receive task. Imagine there is an email with a large attachment needs to send, the sending process starts in the first Send/Receive task, but doesn’t complete when the second Send/Receive
starts. Then Outlook may try to send the email again during the second Send/Receive task, which ends up with two same emails in the recipient’s Inbox.
In Outlook 2013, under SEND / RECEIVE tab, click Send/Receive Groups -> Define Send/Receive Groups. We can then see the window below:
The default interval is 30 minutes, if you think it’s too long you can make it shorter. My suggestion is not to set the interval too short, it should be at least 10 minutes, especially when you have multiple accounts setup in Outlook and always need to send
emails with attachments.
Server Timeouts
This is particular for IMAP and POP accounts. When we have a slow Internet or the latency is high, Outlook may not get response from the Server in time, thus Outlook considers the connection to the server is lost, by default, after 1 minute.
However, if the message has actually been sent in the bad Internet condition, but Outlook doesn’t get a response from the server, it may still try to submit the message in the Outbox in the next Send/Receive task.
We can increase the Server Timeouts to 2 or 3 minutes in this scenario, it will be more probably that Outlook can get the correct response from the server if the message is sent.
For IMAP:
For POP:
IMAP Sent Items options
This is only for IMAP accounts. The recipient may not receive duplicate emails but the sender gets duplicate emails in the Sent Items folder.
In Outlook, there are options for us to decide to save the sent items or not for IMAP accounts.
Outlook 2010:
Outlook 2013:
The IMAP account provider may also save a copy on the Server, if you have configured to save a copy in Outlook, this may result in duplicate. In Outlook 2010, select “Do not save copies of sent items” under Sent Items tab; In Outlook 2013, check “Do not
save copies of sent items” under Advanced tab. Then send test emails to check the result.
Another thing to check is under FILE tab -> Options -> Mail -> Save messages -> Save copies of messages in the Sent Items folder, we can uncheck this option to see the result. What we should note is it’s a global option that applies to all accounts.
If your provider doesn’t save emails on the server, it’s better not to uncheck this setting.
To know if the provider saves the sent emails on the server, contact them or check the webmail settings.
If you have anything to add or any feedback, feel free to let me know.
Please click to vote if the post helps you. This can be beneficial to other community members reading the thread.

Answer 1

Hi,
I found two related links, It might be caused by two or more Outlook clients are logged into the same mailbox at the same time, or some inappropriate configurations in Outlook.
Outlook 2010 sending 2 emails when sending a email. (duplicate email)                                 
http://social.technet.microsoft.com/Forums/office/en-US/8378baff-8c5a-47bc-90a6-8f9c61b2b394/outlook-2010-sending-2-emails-when-sending-a-email-duplicate-email?forum=outlook
Duplicate Messages in Outlook
http://blogs.technet.com/b/scotch/archive/2010/04/14/duplicate-messages-in-outlook.aspx
Hope these can give you some hints , It's recommended to post in this forum if you still have issue about Outlook, they're more familiar with Outlook.
Outlook IT Pro Forum
http://social.technet.microsoft.com/Forums/office/en-US/home?forum=outlook
Yolanda Zhu
TechNet Community Support