In this wacky world of academic research, it's quite normal for people to take a paper which has been rejected from a conference, fix it up a bit, and then resubmit it to a different conference. Nothing wrong with that. The reviews from referees often give useful pointers as to how to improve the results, or explain the problem better, and so on.
This being a small world, it's not impossible for a reviewer to receive a copy of a paper submitted to a conference that they have read before. Again, this is no bad thing: having already read and understood it once, the reviewer has a better insight into it a second time round, and can (provided they keep careful records) compare the two versions and see how it has been improved.
I've just finished reading a submitted paper that I reviewed much earlier this year. It seemed quite familiar, so I dug out the original copy I reviewed. Then I went through the two of them line by line. The two of them are identical. Almost. A few sentences have been cut from the current version, presumably for space reasons. And the introduction has been rewritten a bit. But that's all. Technically, there is no difference between the two. Which is not good, since the paper sucked the first time I read it, and I sent a careful, detailed review pointing out several things that were unclear, a few mistakes, and some general comments. None of these have been fixed. That's rather sad, to expend this effort in an attempt to help the authors to improve their work, and to be met, not with them weighing your advice carefully, maybe following some of it and turning down the rest, but with them ignoring it completely.
Still, I think I know what I will do: I'll retrieve the original review I wrote and send the whole thing back to the authors unchanged. Let's see how they like a taste of their own medicine.