this is waiting for me to accept it, and is the second one tonight which has the same player listed more than once on one of the sides. The first I took duplicate players out of the lineup from both sides and bounced the challenge.
In the challenge listed above player 'procyck' is nominated to play against both 'brickster' and 'bonvieur'.
Is this a bug, or can a clan leader nominate a player for more than one of the matches in the clan game? - I hadn't realised if so!