Yeah, the old copy function next to the map is extremely glitchy, it was basically just a start and is incredibly unfinished, and I just discovered
it copies rooms by reference. (meaning that if you copy a room and then edit either the original or the copy, it'll edit both rooms at the same time). No idea what it accounts for regarding entities, but it can duplicate the start point as well. (Edit: it copies trinkets and rescuable crewmates without changing the counter for them, and it doesn't hesitate exceeding the limit of 20 trinkets/crewmates)
I'll need to change that to use the actually working ctrl+C methods in the background, and as for entities, that'll require a slightly changed copy format (so that entities and their data are copied to the clipboard as well.)
And there is an option for "load from other level"
I've thought about that, but it'd require the other map to be loaded without changing anything of the currently loaded map, and it'd require a separate loading screen and map. I decided it would be best to allow copying rooms while in a level, and using the clipboard so that you could copy multiple rooms and paste them all in a text editor, then loading the level to paste them in.