I think that you would say that this is false.
The actual duel (or tournament) is between Morgan and Sir Sagramore. However, you can look at it as a duel between Morgan and Merlin. Morgan says that everyone in the know was aware that it was really a duel between him and Merlin. The reason for this is that Merlin was explicitly on Sir Sagramore's side. He was giving Sir Sagramore all sorts of magical advantages so that he could beat Morgan.
When the duel happens, Morgan humiliates Sagramore by lassoing him and then finally kills him by shooting him after Merlin has taken away Morgan's lasso by magic.