Restoring mailboxes from backup duplicates messages in POP clients

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?

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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