Same presentation topic for different conferences
It's entirely reasonable to give the same (or nearly the same) presentation for different audiences.  This frequently happens with invited talks, which are often expected to present a survey rather than a particular new result.  Thus, there is no expectation of novelty and thus no issue with plagiarism.

If you are presenting on a paper, on the other hand, the talk should be novel simply because it is about a paper that is novel.