There's a case-by-case issue to analyze here.
First case is a new user to SE as a whole, finds M&TV.SE and posts a question about a sci-fi movie. From there, they find our Sci-Fi.SE site and decides "Hey, I'm going to get the most I can out of this and post the same exact question here too!
- This situation should be dealt by the moderators of the sites, collaborating to find out which site the question is more On-Topic for. You then close the other one and both mods instruct the user to not perform this behavior.
Second case is a new user to M&TV.SE has NO interest in Sci-Fi.SE, has no idea this existed and asks a similar, possibly same question on M&TV.SE that has been asked on Sci-fi.SE before.
I don't expect people, especially new users, to search Sci-fi.SE far and wide to see if their question has been asked before posting it on M&TV.SE (though, I do expect them to search our site before posting). If someone wants to inform the user that it has been asked before on Sci-fi.SE, go right ahead. If that user is interested in those answers, even wants their question closed because they found the answer, then so be it.
However, should they want a different perspective from a site that focuses on just the movie universe or something that doesn't have to be about expanded universes, etc and want answers from our community, that is OK as well.
In this case you are talking about, the user has been told many times by myself and Sci-Fi.SE mods to not cross post their questions.
Not only what I said above, [it is also a policy of Stack Exchange as a whole.
To be as clear as I can be: