Whilst playing recently, my opponent had a nasty bunker shot that, when played, resulted in the ball making contact with the club head twice in the one stroke.

Whilst we both agreed it must be a foul shot - neither of us can find a rule anywhere.

The second hit was unintentional, there was only one swing, but the ball bounced up and was hit by the clubhead as it followed through.

Which golf rule covers this event?

Nic