What prevents someone from turning the same idea into multiple papers? I think the question is fairly clear. Someone could have idea A, and write paper A, but then instead of submitting paper A to multiple journals, he or she might re-write the thing into a paper B, but the basic idea is still idea A.

Say the idea is amazing. He or she could then submit paper A to top journal A &amp; paper B to top journal B, and have two top publications from one idea.

This is obviously wrong, and what I describe is extreme, but what would prevent someone from doing this, what would the consequences be? A bad reputation? 

There might also be less obvious cases, where parts of a previously published paper are re-used extensively. Is something like this ever o.k. to do? Why don't more people do it? Or am I just wrong in the assumption that people don't do this?

PS: I am coming from a Humanities background, I think in some subjects things like these might be more difficult, e.g. in maths or a paper which describes a certain study / experiment.
Submitting (almost) the same paper, the same content, to two different journals at the same time is bad. Once it comes out, the author will be in a lot of trouble, e.g. reputation, both papers withdrawn, etc.

Reusing old results for a new paper is plagiarism if there are not enough new results. Even if the author of the new paper also wrote the old one, it is still bad. There are cases where different papers by the same author look really similar, for example if the author presents in paper (1) a new technique and then publishes paper (2) on nice results obtained with said technique, he might copy some of the explanations on how the technique works. If this is good or bad seems to be opinion based, all I can say is that it happens.

Regarding your question about what prevents people from doing such things:


Yes, a bad reputation might be one possible reason.
Actually, there are people doing this, e.g. trying to submit their paper to different journals at the same time, hoping that at least one of them will accept it. There was a question here a few weeks back from one who got busted doing it and asked for what to do now to save the sinking ship that was his academic career. The reason why it seems that this is not done might be thanks to journals taking measures against it, e.g. not publishing such papers or withdrawing them later on.
One last point to mention here might be colleagues or advisors. If you feel like it might be a good idea to just spam your results to get them accepted or to get a high paper count, you might get the advice that this is not a good idea in the long run from one of the above - if you are lucky you get it before you do something irreversible.