Ah.
It turns out that it’s a terrible mistake to leave the “Autoimport” feature of Photostream on in iPhoto or Aperture when you go on vacation. If you leave it on, and you do the logical thing of moving your daily photographs into your laptop, when you merge the laptop libraries into your main library, you will end up with duplicates of not-quite-everything. By “not quite everything” I mean that your laptop, as soon as it connects to WiFi, probably began sending 1000 photos to PhotoStream.