PRB: Duplicate Messages on Server When Synchronizing OST

• Extended Messaging Application Programming Interface (MAPI), version 1.0
• Microsoft Outlook, versions 8.00, 8.01, 8.02, 8.03

SYMPTOMS

When creating a transport provider, you may want to try to do synchronization with the server like the Exchange Transport does. To accomplish this, you copy the messages to the offline store (Offline Folder Storage - OST). When you reconnect to the server, the messages copied to the OST are copied back to the server resulting in duplicate messages on the server.

CAUSE

The Exchange/OST store treats the messages as though they were created in the OST and then replicates them back to the server. Since the OST is only an ICS (Incremental Change Synchronization) client and not an ICS server, third parties can not access its replication method. Any message created in the OST outside of the OST's synchronization process is thought to be a new message.

RESOLUTION

The only way to prevent duplicate messages is to delete the message on the server after their transport moves them to the OST.

Note that delivering the messages to the OST and deleting them on the server breaks the single instance storage of the messages on the server when the messages are replicated back to the server.

STATUS

This behavior is by design.