As per Microsoft: "This event is caused by having duplicate proxies (firstname.lastname@example.org for example) on two or more objects. Administrators should determine which proxies have duplicates and remove those that are incorrect". See MSEX2K3DB
and the link to "Veritas Support Document ID: 273237" for details on this issue.
From a newsgroup post: "I agree with you that this issue is mostly caused by duplicated system mailboxes. I therefore suggest use the following steps to troubleshoot the issue:
1. Try to delete the duplicated object.
a. Start ADUC snap-in.
b. Click View menu, and select Advanced Features.
c. Expand the Microsoft Exchange System Objects container.
d. Verify if there are duplicated systemmailboxes. If so, please alter the duplicated one which starts with CNF:xxxxxxx.
e. Go to ESM and expand the mailbox store. Right-clicking the Mailboxes icon, select Run Cleanup Agent, and then purge the duplicated one.
f. Clear the current application log and reboot the server.
2. Rebuild the mailbox store in problem.
a. Create a new mailbox store on the server.
b. Move all mailboxes from the original store to the new mailbox store that you created in step one.
c. Remove the original mailbox store.
This new mailbox store has a SystemMailbox object created after the new mailbox store has mounted for the first time. However, if the store that you want to replace contains a very large number of mailboxes, or if the server is running Microsoft Exchange 2000 Server Standard Edition or Microsoft Exchange Server 2003, this method may not be sufficient. For additional information, see ME316622