As I see it from the replay, particularly the last slo mo of it, Mummy jumped into Duncan and, as he did so went slightly side on to his left side with his arm down. Duncan was in a vulnerable position. Mummy as a player had to collide with Duncan in order to stop him from simply running off with the ball. There was a collision. There was no tackle. I think Mummy got off because in the circumstances there was little else he could do realistically. Jumping as he did and going slightly side on were done more to protect himself than injure Duncan. And there is no indication Duncan was more injured by being jumped into or Mummy being slightly turned.
There was a duty of care but in the circumstances Mummy did what a reasonable footballer would do, protect himself and hope for the best for Duncan. Right result. To protect himself Duncan should have known not to take possession of the ball. Presumably he was concentrating on the ball rather than the opposition (to his credit) but lack of awareness of what is coming is will always get a player in trouble. Had he taken the time to assess his options he might have tapped the ball over Mummy in which case Mummy might have had an obligation to avoid or minimise the collision.