If I define acronyms in the abstract, should I define them again in the rest of the paper?
Abstracts are read separately from the paper itself. They aren't an introduction, but more of a condensed version.

So if the paper defines an acronym, it shouldn't rely on the abstract for it. On the other hand, the abstract is supposed to be fairly short, so there isn't usually much of a case for defining acronyms there.

It depends on the paper, of course, but consider spelling out the whole term in the abstract if it is referenced only once there.