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[Hal]

That sounds very impressive!  The one problem I've run into with mail
filtering software is that each message asynchronously spawns a separate
filter process.  This can cause some conflicts with accessing disk files.
I haven't used procmail so I don't know if it has this problem.  But if
so you may need to be careful if there are any cases where two processes
could be accessing the same disk files.  For example, what if two copies
of an identical email message arrive at almost the same time, would your
dup detection work.

The other issue is the possibility of mail arriving out of order.  Looking
for increasing timestamps may cause spurious rejection of some messages.
On the other hand this is a difficult problem to handle in general so
probably the current solution is OK.

Hal