The developer (of the app or the app framework) does have to deliberately implement "merging" of a second launched process. The default would be for the two processes to run side by side in both Windows and Linux. It has been that way forever.

The merging behaviour is also in principle possible for TUIs, btw, it's just that nobody does it. But with something like tmux is would be easy to set up that all subsequent calls to your binary actually connect to the already open session launched by the first instance.