Some people launch Leo from a desktop icon. If you have such an icon and drag'n'drop an outline on it, the outline will open in a new Leo session. Drag another outline, get yet another Leo session.
Some programs (e.g., Notepad++ on Windows) will open dropped files in the same open session. I'd prefer Leo to operate that way. I don't know, programming-wise, how it's done. I presume there's a way to search through the running processes to find Leo if it's there. In Windows, the task manager will list a Leo session as a subordinate to a Python session, so it must be feasible somehow.
However, I wouldn't want Leo to operate entirely as a singleton program. I find many situations where I want to be able to open a second Leo window, and I wouldn't like to give up that ability. In those cases, though, I'm almost always launching the new Leo session from the console since I want to use different command-line parameters.
Terry Brown has some scripts in leo-editor-contrib in which he runs a leoserver server, and clients that send code to Leo via the server to do things like import a file for edit. I'm wondering if some of that capability can be gotten in the way I'm describing instead, without needing to run a server.