Restoring mailboxes from backup duplicates messages in POP clients

[Luis F. V. Gomes]

Hello

I had a disk problem and had to reformat it. All mailboxes were backed 
up using rsync.
After I restored the mailboxes, the POP clients (Thunderbird) that were 
configured to leave the messages on the mailserver for, let's say, 30 
days, didn't understand that some messages were already transfered and 
the users got duplicated messages in their Inbox.
How can we avoid this?

-- 

Thanks

Luís F. Gomes
IT Manager
(55)(21) 3527-1220
Departamento de Engenharia Elétrica
PUC-Rio
R. Marques de S. Vicente 225/401L
22451-900 - Rio de Janeiro/RJ