Most of the Outlook users would have faced the problem of getting duplicate emails or calendar entries. In this article, I have listed down the possible reason for getting the duplicate entries.
There can be many reasons why a user would see multiple messages in their mailboxes. The most likely ones are outlined here.
1. The message has actually been sent more than once by the originator. Check the date in the "Date:" clause. If the dates are different, then the message was sent by the originator more than once.
2. The user is an alias of another account and both accounts received a copy of the mail. To see if this has occurred, open both messages and view the complete header. Look at the last "Received:" clause. This clause will say who the message saw sent to. If there are different addresses, then this explains why the message appeared twice.
3. The user gets mail forwarded from another account and both accounts received a copy of the message. Use the same check as above to see if this is the case.
4. One mail server between the sender and recipient is (incorrectly) duplicating the message. To see if this has occurred, open both messages and view the complete header. Match each of the "Received:" clauses until you find two that are different. These two received clauses will give a different time for the receipt of the message. This clause identifies the server that is duplicating email messages. You will need to contact the owner of the server for further investigation.
Other items to help determine the cause of the duplicate messages appearing include taking a look in the headers of the messages themselves. The topmost Received header will contain a unique ID for the message consisting of 8 characters.
You can then take a look in the SMTP log for the server and search for this unique ID. This will show you the exact SMTP transaction that caused the message to enter your server. You will be able to see exactly which users the message was delivered to.
If the unique ID's are different then the messages are the result of more than one transaction. Likely reason being that the message was delivered to the server more than once. Again this can be confirmed from the logs.
When trying to debug these issues it is helpful if you enable all of the logging options for the SMTP service.
Reasons why you might see a message being sent more than once can include
1. Misconfiguration of the sending server
2. High CPU usage on your server for extended periods of time, causing connections to time out and thus the remote server retries the message again.
3. Malformed content being sent by the remote server
If you use the Anti-spam option to scan content for restricted words and do not limit the scanning to a certain number of lines, you are likely to see periods of high CPU usage on your server, particularly if you frequently process messages containing large attachments.
Hope the above detail is useful.