I've noticed that the Exchange Mailbox Merge (Exmerge) utility can make copies of single-instance messages for delivery to multiple mailboxes. How does Exmerge determine which messages to duplicate?
At first I assumed that Exmerge used the message ID field, but the message ID isn't always the most reliable way to identify related messages; the sending server can put anything it wants into that field. Microsoft Developer Network (MSDN) documents a Messaging API (MAPI) property, named PR_SEARCH_KEY, that Exchange uses to correlate related objects. Exmerge uses this property to identify messages that need to be duplicated.