In some rare cases, the conversion tool may not be able to convert source mail files to a Vaultastic compatible format. In such a case the mail will be skipped.

If we resubmit the request, will there be duplication on the destination server?

If a failed request is restarted, there will be no duplication. However, if a processed request is resubmitted with the same destination folder, then there will be mail duplication on Vaultastic.