We encounter the following error message in one of our Exchange 2007 to Office 365 using Direct Cut Over Migration Method
UserDuplicateDiscoveryException: The user "firstname.lastname@example.org" was discovered twice from the source environment. Please ensure that there is no duplication in the source.
We had checked there is no Duplicate ID in Exchange 2007 & AD, and finally the root cause of this issues is that there was same users in the migration job that we created
We removed the duplicated users in Migration Batch using the command below and perform a resync again. The Error message disappeared this time, and the user are synced successfully.
Remove-MigrationUser -Identity email@example.com