If you try to launch Everest while the game is already running it gives you this prompt as expected:

Which says "Do you want to launch another instance anyway?" however when you press launch it says Everest is opening in the background and then it opens the same Celeste window that was already running.
It's possible that it might be a Windows 10 issue specifically, or simply that it's not intended to launch another instance and the text is wrong
admittedly this is kind of a niche issue