I cracked a (brand new) right side rotor at the track - should I replace both?

they are just cheap ones from Carbotech I'm not sure I can find exactly the same

 

One at a time is fine.

 

Usually.

Just be prepared for a few unusual pulls and vibrations and stuff until the new rotor gets itself used to the pads. Don't expect to go out and drive 100% the first laps.

 

one at a time is fine.

treat it like new rotor (duh) and don't expect great braking for some laps. tho this doesn't matter if you change one or two.

 

Of course, there is always the question of "if one rotor failed, and the other rotor has an identical history, how soon will it be before it fails?"

Be ready. It might also crack soon ... or it might not. Many possible variables.

 

Whats weird is I think it happenned after the last session when the car was sitting because I don't remember feeling anything on the track.
I came off and the car sat maybe 30mins while I packed up then when I left I felt a nasty vibration from the front on the brakes.

I have ducts and I'm pretty sure I did a decent cool down lap

 

That's not weird, that's normal. Almost always the rotor cracks when it is cooling in the paddock.

 

This has happened to me my last two events- both times it was the right side. I don't have any doubt that despite being ducted, Hoosiers and XP12's on front have a lot to do with it. That, and the fact that ambient temps have begun to fall here. At VIR I changed both because it looked like the driver's side was about to go, too. This past weekend at Summit, I changed the one to get on track and then later elected to change the other.