Ok, first of all I want thank everybody for their input. It's much appreciated.
However, I just watched the Vera Cruz clip on YouTube and it's not the film I'm looking for. In Vera Cruz, Lancaster is shown twirling his gun from a long distance, but in the film I'm looking for, the guy is shown much closer and there is even a closeup of his hand twirling the gun. Also, the look and feel of the movie was not as old as Vera Cruz. I'm guessing it was from the seventies. Also, the duel happened in a town's main street, not a bullfighting arena or anywhere like that. I seem to remember the 'good' guy and 'bad' guy being old friends but on the opposite sides of the law. Hope that helps.