Was it just a coincidence that 1492 and Christopher Columbus both came out in same year? The different movie makers must spy on each other and try to outdo the other. That is why you just happen to have two movies about the fight at the OK Coral released at the same time. Both Tombstone and Wyatt Earp came out in 1993. Maybe they say "I heard another Studio is planning a movie about a volcano. We are going to miss out on the volcano craze if we don't compete". In 1997 Dante's Peak and Volcano are both in theaters at the same time.
Perhaps the rivalry is as simple as getting a short e-mail challenge that says: Lame Mars science Fiction movie due in 7 months! Then in 2000 both Mission to Mars and Red Planet are released. I saw Without Limits in 1998 which is a movie based on the life of long distance runner Steve Prefontaine. When I returned the video I noticed a movie called Prefontaine which was basically the same story that another studio had released the prior year.
What are the odds of having more than one action packed drama released at the same time about a meteor headed towards earth that will wipe out the entire planet? Deep Impact and Armageddon were both in theaters in 1998. How about a gritty epic World War 2 movie with all star casts? Saving Private Ryan and The Thin Red Line also came out in 1998. For the longest time I honestly thought that Antz and A Bug's Life were the same movie. Apparently they are separate animated ant cartoons that both came out in the same year.
In 2006 did you see a suspense/mystery/drama about Magicians in the early 1900's in Europe? I have actually heard cases of people going to the theater and seeing the wrong movie because their friends weren't specific enough about which Magician/period movie they were recommending. The Prestige and The Illusionist were both good movies and even better examples to illustrate this topic.
The saddest form of copying is when a big movie is coming out and another studio throws together a cheap piece of junk movie that goes straight to video with essentially the same name and storyline with hopes that the uninformed or very elderly will not notice the difference at the video store.
If you can think of any other movies let me know by listing them in the comments section. Ideally they should be released within a year of each other. Also sequels obviously wouldn't count. I know there are many more out there, so let me know the next time you see another example of this.