A recurring problem with multi-copies

[Harry Putnam <reader@newsguy.com>]

I've never fully understood all the variables related to handling
duplicate messages, there may well be some setting to handle this
recurring problem.  I could use some advice as to how to handle it.

Problem:

I sometimes copy messages to topic named (nnml) groups that I want
to keep for reference.  So sometimes a thread occurs in a newsgroup or
mail list group where I want to keep parts of the thread.  An example
might be in comp.lang.awk.  I post a message (query) and begin
recieving replies.

I copy the usefull replys to nnml:t.awk.   Three weeks later someone
who was on vacation replies to my query with really good or
controversial information.  Another round of replies start, many of
these I also want to keep but by now have forgotten what I kept and
what I didn't.

So I end up with a number of duplicates in nnml:t.awk

I'd like some built in (or add on) way where gnus would tell me that I
already have copies of the dup messages in `t.awk' group, and asks if
I want to copy them anyway... or something similar.

The best would be a situation where gnus would ask if I want to
continue copying and give me a choice of  only the unique messages or
dups and unique.