parsearchive: Handle duplicates

parsearchive: Handle duplicates

The parsearchive tool can be used to load missing messages sourced
from mailman or another source. In this use case, there's a good
possibility that at least some of the messages found in the archive
are already stored in patchwork. Handle this case by ignoring these
duplicates.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>