Electron requires effort to get single instance. I've had to implement it. You need to call this:

https://www.electronjs.org/docs/latest/api/app#apprequestsin...

and handle this event:

https://www.electronjs.org/docs/latest/api/app#event-second-...

I believe macos is the only OS that enforces single instance by default. If you start xterm five times on linux, you get five independent xterm processes.