I don't think you should be too paranoid about this sort of thing, at least not here. True, it does happen, but you can't expect to put ideas out there and not leave an impression on somebody, on some degree. If you really argue about it, they could just say that your idea "inspired" them, and you can't really contest that. How many plugs have been inspired by legally copyrighted material, such as Babylon 5 or Star Wars? It's clear who had the idea first, perhaps, but you'll both take it to a different end result anyhow, and the plugs may then appear similar, but certainly not identical.
If it's an engine idea especially, it's all in how you connect it to the story in the game. In this way, some clever Control Bit trick could be hijacked by three other people, and none of the four plugs would bare any resemblence.
If it's a graphic or a ship design that someone copied, it's all in their attitude about it. I made a ship a while back called the "Apollo", which I myself made three versions of, and I believe four other people have made similar ships after seeing mine, inspired by mine. I think that's neat, personally, as well as flattering. If they ripped a sprite right out of your plug, though, it will be widely known, and they couldn't survive here by trying to take credit for plug graphics already released.
...At any rate, as far as I understand it, this developers board is just as much for sharing ideas as it is for sharing knowledge and asking questions. I like to trust in people that they won't try to take credit for something somebody else publicly posted, but whether or not they use that idea or play around with it is entirely up to them. It's all about taking credit you don't deserve, i guess.
But, if you are paranoid about it, avoid telling the world all your ideas, by all means. Like EVula said, loose lips sink ships.
------------------
(url="http://"http://mailto:ckey@integraonline.com") - () () / }{ - (/url)
where...
...am I?