I remember...i guess it depends how "main" your idea was...is your sitcom a flop without it...is there a way you can change it at all? But i expect you've already thought about this. Thing is -no one's going to take up a sitcom that's already been done. Maybe email it out to some people...give them a chance to read it and see it from a different perspective, then make your decision! 
I'd love to just tell everyone what the idea was, and which show had used it, so you could all give me your opinions; but there's no point in me doing that, as I might just make the situation worse.
I had the same problem after sending a pilot to a certain TV company. four gags turned up in other programmes. If its not word for word, i'd carry on with it.
Good luck.
Yeah, I am probably going to carry on with it. I can't bear to scrap it completely, but I will probably have to rely on it less than I'd originally planned, in order to get laughs. The frustrating thing, is that I came up with the idea around the time of the World Cup, in June/July, last year.