Causing duplicate-message checking to ignore certain groups?

[Lloyd Zusman <ljz@asfast.com>]

I'm using a fairly recent version of Oort.

In most of my nnml groups, I have a "Gcc:" header which causes messages
to go into a special archive group.  For example, when posting to this
mailing list, the header looks like this:

  Gcc: nnml+archive:gnus

I also have a different nnml group into which this mailing list gets
directed.

And finally, I have the following setting:

  (nnimap-treat-duplicates 'delete)

I'm sure that you can figure out what's this leads to: when I post to
this group, a copy of my message immediately gets saved in my
"nnml+archive:gnus" group, and then later, when the message I posted
here gets sent back to me, the duplicate-message checking causes it to
be deleted from my regular nnml "gnus" group, and I don't see it.

In general, I want to avoid seeing duplicate messages, except I'd like
my "archive" groups to be ignored during this duplicate-message
checking.  Is there a way to do this?

Thanks in advance.
I 
-- 
 Lloyd Zusman
 ljz@asfast.com