Like many others who have no doubt gone before me, I found myself spending a large chunk of time this week finding and merging duplicate contacts. Civi's deduping UI has gotten a lot better lately, and I love having the ability to go through it more quickly than before. But deduping is only as good as the queries it runs, and of course prevention is better than cleanup.
Here are some observations of how all those dupes in my DB got created: