Problem moving to new Exchange server

I have an existing Exchange 2000 server (Windows 2000 AD domain).
Recently installed and configured Exchange 2003 onto the network. When I move a mailbox from the old server to the new server, it moves with no problems.

The mailbox account can send email without a problem. When someone from within the exchange org sends that mailbox an email they get the following delivery error:

Public Information Store (Server)
A configuration error in the recipient's e-mail system prevented delivery of this message. Two recipients are configured with the same e-mail address.

I did wait several hours for changes to replicate throughout the domain and the problem still exists. The message is not delivered. When I move the mailbox back to the original server, email services run normally - although the previously rejected emails never come through.

I have run some quick Google searches which suggest there is a public folder with the same user name. We don't use our public folders.