(I know this is old, but here might be an answer. It’s not the right answer, but it explains something).
As far as I know, there is no way to accurately deal with this. I myself have needed a solution for this, but both options are marred.
First, Unity 3 Standalone’s on Windows platform are -SUPPOSED- to have a command line that you can type in to force the game to only run one instance. It’s as follows:
Theoretically, if the game is started again, it will find the current process and switch focus to it instead. However, I haven’t got that command line attribute to work, or any other Windows ones (Such as -popupwindow). So I went on to try something else…
The System.Diagnostics.Process namespace in .NET contains a lot of nice little goodies. Such as the ability to search currently running processes on a machine for one of a particular name (GetProcessByName), or give a list of all currently running processes (GetProcesses) so you can search if your Game’s process is already running, and kill the game before THAT version loads.
However, the Mono version currently included in Unity is actually an older version, which contains a bug. So any use of those two aforementioned functions within System.Diagnostics.Process will give Exception Errors for no reason.
Using GetProcesses() returns an array of currently running processes, but getting anything but the PID of the process returns an Exception error, saying the process has exited (But it hasn’t).
Using GetProcessByName() won’t go anywhere, either, and gives the same error.
So… In short, I’ve found out that you cannot check if your game, or anything else is running.