Can I have a send hook dependent on *two* addresses in the To: ?

[Chris Green]

I want to filter out some cases where E-Mail gets sent to two addresses
and should only be sent to one address.

E.g. I have a mailing list where for various nefarious reasons when I do
a L[ist reply] the To: line is as follows:-

    To: ix...@ixion.org.uk, ix...@ixiemaster.ixion.org.uk

So can I use a send-hook to replace the above To: with a corrected,
single destination, To:  ?

-- 
Chris Green