James Cagney vs William Cagney
Man, I was completely taken in by the guy who played the McSwatt role. I always assumed all these years that it was James Cagney, in fact I even tried to look it up in all his IMDb-listed roles and couldn't quite understand why it wasn't there.
It's only today I learn it wasn't Jimmy after all, but his brother William. Talk about look-alikes!