I want to repair this damage done to Subject lines, notably in mailing
lists which have standard tokens in them.

For example, and as I'm sure everyone knows, you get a mailing list
which automatically prepends "[foo]" to every subject line if it is
not already present:
    Subject: [foo] the original subject
Normal followups to this, in the absence of other hackery, become:
    Subject: Re: [foo] the original subject
Egroups and related list-support sites will leave this one alone,
because the needed "[foo]" token is in place.  But then we Gnusers
come along, knowing that we don't need no steenkin' subject tokens,
and so we strip them out:
    (setq nnmail-list-identifiers '("\\[foo]"))
so that what we see is:
    Subject: the original subject
and when we reply, we generate:
    Subject: Re: the original subject
List handlers decide this is inadequate, so they re-append the token:
    Subject: [foo] Re: the original subject
and then some non-Gnuser follows up, generating
    Subject: Re: [foo] Re: the original subject
which the list-identifier-stripping Gnuser will subsequently see as:
    Subject: Re: Re: the original subject

The patch below addresses this during nnmail-list-identifiers
processing, detecting repeated occurrence of "Re: " and reducing it to
just one.

--karl