Multiple messages received by Exchange client

Link:

http://support.microsoft.com/kb/143375

Summary:

There are instances when an Exchange Mail client receives multiple copies of the same message that is sent from the Internet via the Internet Mail Connector (IMC) or by a Microsoft Mail 3.x client via the Microsoft Mail Connector.

More Information:

This behavior occurs if the Exchange recipient belongs to multiple Distribution Lists (DL). For instance, if user1 belongs to Distribution List 1 (DL1) and to Distribution List 2 (DL2) and an Internet or Microsoft Mail 3.x client sends to both DLs, the Exchange client receives two copies of the same message. If DL2 is a member of DL1, then the Exchange recipient receives three messages. 

This behavior is by design. It is an X.400 feature. Exchange Server tries to eliminate duplicates, but for mail coming from external sources, the information that is required to accomplish the task is not available.