What? 99% of my programs I can open more than one window by just executing the program as normal multiple times, the only ones that enforce a kind of singleton execution are things that also acts as a kind of server, one example is Everything search, but it has a "Open new Search window" for multiple windows. And that is the default, you have to explicit code in a lock/global mutex thing to block multiple instances.

vscode, electron app, no problem opening multiple windows. Geany, gtk app, no problem opening multiple windows.

Can you give an example where this doesn't work?