Can't find anything in settings
There is never more than one dopus.exe process running at once, but if you want to restrict things to a single window then see here:
It works only in part. For example, if I open file explorer from another program, for example visual studio - right click on project - open folder in file explorer, it still opens a new DO lister. This can't be changed?
Preferences / Launching Opus / Explorer Replacement / Open external folders in a new tab from the linked guide covers that.