Xcode 9 GM seed installation

It used to, but now, I think if you just unpack it next to an existing version named Xcode, it will rename itself to Xcode 2, as an example.

Best practice, tho if you want to keep an earlier version...rename 'Xcode' (8.x) to 'Xcode 8.x', 'Xcode 8.3.3' whatever, first, so you can better keep track.