It happens all the time. You think you just came up with a brilliant idea, you present it to the creative group and someone says "Yeah, that's great, but I've seen it." Then it's back to the drawing board. Sometimes, two different ad agencies are working on the exact same concept for a similar product at the same time. Projects can get killed because another Company launches the same campaign idea first.
Coloribus is a fascinating (and slightly depressing) site that has collected hundreds of examples of the same ad idea done in slightly different ways. It just goes to show you that original ideas are few and far between. Here are some highlights to check out: Ducks and Horse Power