> use the good ol procmail rules: (see "man procmail")
>
> :0 Whc: msgid.lock
> | formail -D 8192 msgid.cache
>
> :0 a:
> duplicates
>
>
> Then dump the mailbox through procmail (make sure that nobody will try
> to deliver to the mailbox while you're doing this...)
>
> formail -s procmail < mailbox
>
> You probably want to be careful what other procmail rules you have in
> ~/.procmailrc, since all will be applied. You probably want to redirect
> the output of formail somewhere, as well...
Not that I'm procmail literate (or so I'm told), but put the above
recipe in a file, e.g. rc.filt, and run
$ formail -s procmail -Y -m ./rc.filt < mailbox
