Duplicate detection of delivered messages is done by the Exchange store. The store does duplicate detection based on two properties on the message - the Internet Message Id and the client submit time. We would have liked to do duplicate detection based solely on the Internet Message Id but there are several not-to-be named applications out there that use the same Internet Message Id on all their messages.
The store keeps track of duplicates using a table in JET called the DeliveredTo table. When a message is delivered to a user, the store checks this table and if no entry is found the message is delivered to the user and a row is added to this table to indicate that the user received the message. If an entry is found, the message is turfed.
The store only tracks duplicates for 1 hour by default. This can be changed by changing the value of registry setting:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Server Name>\<Private/Public-Guid>\Track Duplicates (in hours)
The maximum value that the store will accept for this registry value is 49 days, if a value greater than 49 days is set, the store will ignore the value and keep duplicates for 24 hours. But keep in mind that increasing this value will cause this table to grow really large and this could slow down delivery.
The store will periodically delete the old items from the deliveredTo table which is handled by the background cleanup thread which runs every hour. This is also configurable and the registry setting is:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Server Name>\<Private/Public-Guid>\Background Cleanup (in msecs).
There is still a chance that you will get duplicate email if the delivery of email is delayed because of the following reasons:
1) If either the internet message id or the submit time is different on the two messages, the second message will not be treated as a duplicate.
2) If the two are the same, but time interval between the arrival of the two messages is greater than 1 hour, the store cleanup task would have deleted the original entry in the deliveredTo table and the user will get duplicates.
3) If the user is moved. The deliveredTo table is per database and the information is not moved when the user's mailbox is moved.
4) In older versions of Exchange we had a problem where duplicates would occur when a message was sent to a user and a DL containing the user using OWA. When the message was submitted the store would stamp an Internet Message Id on the outgoing message. However, since OWA submits messages with native MIME and the fact that the Internet Message Id stamped by the store on submission did not update the MIME Message Id header, the MAPI message was out of sync with the native MIME. The message would then be bifurcated by Transport and this would result in messages with different Internet Message Ids and therefore, duplicates. We changed this in Exchange 2003 so that the store only stamps the Internet Message Id on a message if it detects that the MIME has to be regenerated or if it is a pure MAPI message.