As I've discussed before, Hollywood has a habit of simultaneously releasing two movies on the same topic ("Volcano" and "Dante's Peak," "Deep Impact" and "Armageddon," "The Truman Show" and "Ed TV.")
Maybe studios have gotten better at avoiding this pitfall, because I can't think of too many recent examples. "The Illusionist" and "The Prestige" was a doozy, but that was four years ago.
Now I'm wondering if "Megamind" and "Despicable Me" fit the bill.
They're both about bald supervillains with troubled upbringings and henchmen called Minions. They both learn to become good after finding people they care about. Then they ultimately have to defeat a truly evil foe. (Not to mention the fact that the movies are both 3-D animated films.)
Also, the main characters were both voiced by actors from "Anchorman." And Kevin Bacon wasn't in either movie. [You can stop now. -ed.]
UPDATE: I forgot about another more recent example than "The Illusionist"/"The Prestige."
"Paul Blart: Mall Cop" and "Observe and Report" (both in 2009) were about as close as ideas get (even if the execution was different). I even blogged about it here. Maybe studios aren't getting any better at this.