It happens over and over again: two movies will come out in the same year, or close to it, that have the same subject and often the same plot. The most memorable pairing was Armageddon and Deep Impact, which both came out in 1998 and told the story of destroying an asteroid/comet before it hits earth. Both movies were hits. A more common story with twin movies is that one vastly outshines the other, and years later we forget that they were actually different movies. Sometimes it's due to what's interesting to the audience of the time, sometimes it's one film trying to piggyback off the expected popularity of the other, and sometimes it's pure coincidence.
NPR put together a list of 50 pairs of twin films, although a few may be arguable. While both Lincoln and Abraham Lincoln: Vampire Slayer came out in 2012, they will never be confused with each other. And two movies about Christopher Columbus coming out in 1992, the 500th anniversary of his most important voyage, is not much of a coincidence. There are more such pairings in the discussion at Metafilter.