As Scott discussed the other day at Go Into The Story, it happens fairly often that a writer will discover that an idea they've been laboring over has just been written and sold by someone else. Most of the time, this might render that writer's idea useless for the time being. After all, if Paul has just sold, you're probably not going to want to send around your comedy about two geeks who meet and go on a road trip with an actual alien.
However, it's not too unusual for some writers to attempt to get traction on their "dead" scripts, either by entering them into competitions, or by taking advantage of whatever limited connections they have to get them read by people like me. In that case, it often falls to a reader like me to say, "Hey, this reads like a rip-off of [insert movie that beat them to the screen.]"
If that happens to you, here's what NOT to say, "Yeah, but I came up with this idea in 2004 and was working on it WAY before those guys sold their script! I didn't rip them off!"
In the immortal words of Tommy Lee Jones' Lt. Gerard... "I don't care!"
It doesn't matter if you thought of it first. Your script is still useless. Dead. Finito. (Unless you find someone who WANTS to make a rip-off.) All that matters is perception. If I as the reader have seen that this idea has been done before, you have already landed in second place.
My bosses are not going to pay good money just to re-do I Now Pronounce You Chuck & Larry. Sorry, bub.