Billy Crudup x 2?

Watched this film again tonight, for the first time since it came out. When we met Julia Roberts' husband, Greg, in the film, I thought, oh there's Billy Crudup. Next thing, there's another character called Ken who's in the airport and the back of the taxi and I think, there's Billy Crudup again, is he playing two characters? After seeing the credits, I see the former is Robert Knepper and the latter Mr Crudup- but they look so alike! Is it a good idea to cast people who look too alike in the same film (unless of course it's plot or they're related)?

The French haven't the nature for war. They'd rather eat and make love with their faces than fight.