Interaction of reformail -D with EX_TEMPFAIL

Hello!

I'm using maildrop, and I have lines early in my .mailfilter which
read:

# Drop duplicate emails
`reformail -D 10000 $HOME/var/maildrop/seen-msgids`
if ($RETURNCODE == 0)
{
  to "Mail/spool.duplicates"
}

which should work great.  For some reason, messages are occasionally
being rejected with error code 75 (EX_TEMPFAIL) for no obvious reason,
and are then successfully delivered on a second attempt.  But one
consequence is that they are passed through maildrop twice, and they
got at least this far in the .mailfilter file the first time, because
the second time through, the msgid has already been seen, and so the
message is regarded as a duplicate.  (Good job I'm saving these
messages rather than discarding them!)

Is there any straightforward way of handling this strange situation
with maildrop, that is, to ensure that an EX_TEMPFAIL doesn't leave a
trace in the seen-msgids file?

Best wishes,

   Julian