MC actually looks up
https://www.themoviedb.org/ for movies. It just happens to return the "IMDb ID" from the lookup as well.
If you are selecting multiple movies and running the "Get Movie & TV Info" function, I believe that MC picks the best match it can find, and uses that. If there are movies that have exactly the same title ( [Name] tag ) I think it uses the [Year] tag, if known, to try to pick the best match.
For example I have the movies "The Day the Earth Stood Still" from 1951 and 2008. If I do a lookup on either one, it finds the correct one and lists it first in the list of results MC returns, and lists the other version second. In fact it lists a third option "Giant Robo: The Day the Earth Stood Still" from 1992 as well. But if the year is correct, it selects the correct movie.
If I select both versions of the movie and run the "Get Movie & TV Info" function on them both are still given the correct information, because they already have the correct [Year] tag, and are therefore the best match.
You can spot check which movie will be selected by running the "Get Movie & TV Info" function against just one movie, and check the list of returned results. The one that is going to be returned if you run the "Get Movie & TV Info" function against multiple movies is always going to be the first on the list of results.
But if you want to lookup multiple movies at once, you are going to have to get the [Year] tag correct first. As always, check this with just a few movies before you do a whole lot, just in case something doesn't work right. Back up your library before you start as well.
BTW, I also tried changing the Search Criteria in the "Get Movie & TV Info" function to just the "IMDb ID" to see if that would force selecting the correct movie. It doesn't. I don't think the lookup takes the "IMDb ID" into account. It just returns that value along with other metadata when it finds a match.