There is a case where a message can be reflected back to the listserv and since
you approved it already and the headers contain that information, it gets back
through to the list.

If you search the archives for "Mailer security problem" and "Duplicate Posting
Problem",
http://peach.ease.lsoft.com/scripts/wa.exe?A2=ind0103&L=lstown-l&P=R1700&I=-3

you'll find references to the anomaly.

 >I recently posted a message on our newsletter list. The posting failed
 >to include the Subject-Tag and did not process the Top_Banner template.
 >It has included these in all other postings for the past 2 years. What
 >would cause the sudden failure?

Randy