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Consider the following scenario. An Exchange 2007 user sends a message to a parent distribution group that includes child distribution groups. More than one child distribution group includes the same one-off recipient. In this scenario, the message is delivered multiple times to the target system of the one-off recipient. If the target system does not detect the message duplication, the recipient receives multiple copies of the same message.
Note A one-off recipient is a recipient whose address is not in the Active Directory.
This problem occurs because the Exchange Server 2007 server uses the Exchange store to detect duplicates. However, this method does not work for one-off recipients.
To resolve this problem, install Update 8 for Exchange 2007 Service Pack 1.
For more information about Update Rollup 8 for Exchange Server 2007 Service Pack 1, see the following Exchange Help topic: