One minute I am positive it's his left...then the next I am positive it's his right....
This sounds the same as right before Lacey tore her ligament in her knee. We thought she was limping in what we thought was the front, and couldn't make up our minds which side, and next thing you know she was chasing a ball and down she went and on to surgery.
Don't know if that's it but it sounds exactly the same, so I had to comment. I hope you find out soon what is causing Duncans problem.