Getting this as well with Xcode 7.
I've been following these simple steps to avoid seeing the error:
Annoying, and not a solution, but I think this is a bug within Xcode 7.
Completely quitting Xcode 7 and reloading resolved the simulator is already running it for me.
simulator is already running
I got this error too, and building and then running the app (cmd+b ; cmd+r) solved the problem.