@Allosaurus said:
A proposition: Allow adding a movie to multiple collections
Hello! I think it'd be very good to have this ability.
There are a lot of movies that are crossovers, for example, between character A, which has its own collection and character B, which also has its own collection.
There aren't even any guidelines as for what to do, when these characters have equal roles in a movie.
To which collection should they be added?
That's why I propose to have this function.
If needed, I can even give a lot of examples for when it's necessary.

Originally this collection feature was only used in Media Center, originally in XBMC now Kodi.
Plex is based on XBMC.
And I believe that Kodi and Plex do not accept that a Movie belongs to two or more collections.
This logic was also used here on TMDb. I don't know if this change is too complex.
Anyway, there are plans to add the Universe feature.
Maybe that solves the problem. See the item on the To-Do List on Trello.
https://trello.com/c/cRiMnOvH/79-create-universes-franchises

I know that there are plans for the Universe feature. But this is different. I don't know if it's hard or not to make this change, I just do what I can – proposing it.