Duplicate (triplicate) messages in inbox for IMAP user
Problem reported by Randy Armbrecht - November 4, 2015 at 7:39 AM
Submitted
We have a strange issue going on. We have one single user on a domain on our server that has his email client (Outlook 2013) setup as IMAP to access the account.  He receives randomly duplicate or triplicate copies of messages into his inbox.  We have looked at the logs of the incoming gateway servers and the SMail logs, and both only show delivery of a single message.  Pulling our hair out trying to figure this one out.
 
Anyone else seen or dealt this this issue in the past?

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We had a similar issue, and we found out it was a graphic driver. We removed the graphic driver and suddenly the issue was resolved. It took us about 3 days to figure it out .... Not saying it could be your problem.
 
When you setup the account on the computer and do a test, does it show 1 or several tests?
 
Here it showed 2 tests when creating and the testing the account.....
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First of all, make sure that there are not duplicates in their webmail. If there are no duplicates in webmail then you know it is a client-side only issue. I've seen this happen commonly with one of these issues client-side:
 
  1. The IMAP interval is set too frequent (i.e. less than 15 min.) and if an IMAP session takes too long to synchronize their Inbox a new IMAP session will be triggered by their email client, resulting in duplicates or possibly even triplicates (you commonly see this on Monday mornings with high-traffic accounts that have thousands of new emails or just a few messages all with large attachments). In such a case every message won't be duplicated, but only some of the messages (as the second IMAP connection begins with the first unretrieved message which is currently being downloaded by the first IMAP connection).
     
  2. The IMAP client is not sending confirmation after having successfully RETR a message, resulting in it the same message being served by the server upon their next connection and redownloaded again (and possibly again). This happens when there is network latency or dropped connections (especially common on cellphones while moving between towers).
     
  3. Client-side Anti-Virus scanning on Mail causes enough latency to allow items 1 or 2 above to occur. Temporarily disable AV on their computer to see if duplicates are no longer occurring.
     
  4. Email is delivered to an account or alias that forwards to multiple local addresses and the email client is retrieving email for more than one of those addresses. (i.e., messages to support@domain.com are retained and also forwarded to employee1@domain.com, employee2@domain.com, employee3@domain.com. Their email client is set to retrieve email from support@domain.com and employee1@domain.com for example, getting two different copies of the same email.) In such a case they aren't technically duplicates and nothing is wrong, other than they only need to IMAP the end account (employee1@domain.com in our example instead of both).

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