Unrestored items after successful migration

Hello,
we have just followed this howto to migrate a 6.0.11 32 bit server to a brand new 7.1.2 64 bit server without problem. But at step where a few mail are missing (the new server is in production) , we tried to import a mailbox, and saw that all mails where imported 2 times ! what are we doing wrong ? maybe because we renamed the server ?

Thanks and greetings from france.