Humble Indie Bundle 3?

Humble Indie Bundle 4 included a copy of Humble Indie Bundle 3 so that would explain why a lot of people have duplicates. I actually have a key of it myself but there's a chance I gave it to someone and forgot so I haven't given it away =[

They (and by they I really mean 1 person that I've observed thus far) acquire them in truckloads when the bundle was around and save them for later
Or as mentioned by KaBob some bundles do give extra copies of the earlier bundles

I believe this was back when you could buy bundles with Steam keys for 1 cent. People were hoarding them to make alt accounts on Steam to mine for coal during that winter sale.

There is also an 'aftermarket' for bundles. There are stores still selling them.