List reply is sending to two addresses - how to fix it?

[Chris G <cl@xxxxxxxx>]

I am a member of lots of mailing lists, most work fine with the L[ist]
reply command (having added them in the right way to my muttrc of
course).  However one list has recently started getting *two*
addresses in the To: field when I hit L[ist] reply.

When I do a L[ist] reply the To: comes up as:-

    To: ixion@xxxxxxxxxxxxxxxxxxxxxxx, 'Ixion' <ixion@xxxxxxxxxxxx>

The address ixion@xxxxxxxxxxxxxxxxxxxxxxx appears only in:-

    X-BeenThere: ixion@xxxxxxxxxxxxxxxxxxxxxxx

The address 'Ixion' <ixion@xxxxxxxxxxxx> appears in Cc: :-

    Cc: 'Ixion' <ixion@xxxxxxxxxxxx>


So how can I make mutt only reply to one address, it's getting a bit
of a pain seeing two identical messages every time I send!

-- 
Chris Green