This isn't about movies with twins, but two movies that have similar themes released at the same time.
In 1998, Antz and A Bugs Life were released within 6 weeks of each other. There is a storied history of how Dreamworks and Pixar raced to theaters with their ant themed movies, but this is one example of a much larger phenomenon. There have been multiple occasions where two identical movies arrive in the same year.
This came to my attention during my flight back to Toronto from London. On the plane I watched Bohemian Rhapsody, a film about the life of Freddie Mercury, a charismatic musician struggling with his sexuality. The next day I came across a trailer called Rocketman, what looks like it's going to be an amazing film about the life of Elton John, a charismatic musician struggling with his sexuality.
I remember noticing a lot of disaster movies in the years leading up to 2012, which made sense to me. Some people believed that the world would literally end in 2012 so why shouldn't Hollywood profit off of that? I thought that I would be able to find a large spike in the number of natural disaster movies during this period, but after finding a list of every disaster movie, I've concluded that humans just really like disaster movies.
That being said, why do twin movies get made? I believe it is because the studios are trying to predict what the future zeitgeist is going to be when the movie gets released. Predicting the future, or the "spirit of the times", is quite hard, so it makes sense that different studios will come to the same conclusion given the same incomplete data about what the future zeitgeist will be. More often than nought, these twin movies are developed completely independently from each other because both are a product of times.
Another reason might be copyright and the public domain.
"For example, a studio knows Dwayne Johnson is making a Hercules movie, and they go 'Hey, we have the rights to this other script to another Hercules movie, and the legend of Hercules is in the public domain.'
Trying to beat other studios to the theaters would explain twin movies, as a race to capture a story first.
This is a great example of many twin films, with a quick summary of what each one is and how they are similar.