It's inevitable that some people are going to have similar ideas for levels.  I have several ideas jotted down right now and, while I think they are relatively unique/creative, I really can't be sure because I haven't played any created levels yet.  I want to finish story mode first.  Since it's virtually impossible to have played every single available user created level, many people may have absolutely no idea that 1 or more of the ideas incorporated into their level has also been used by someone else.  Do you think most of the community will be aware of this reality?  I can envision some sour grapes by user A when user B unknowingly makes a similar level that is superior.

I wouldn't worry about it. It's all part of the game. I'm sure plenty of people will be tweaking other peoples creations

On the flipside, it could create an interesting vibe of friendly competition.

Yep. That's the idea I guess. It's one way of getting to know people.

I doubt it, you can make whatever you want only with the available tools, so it is limited to what you can make.

No more so than in the actual paying industry. That's life. You come up with an idea somebody makes it better. Maybe they copied you, maybe they are just a better human being.

 LOL.  Good point.  Legitimately original ideas are few and far between anymore.