Comment 17 for bug 1263540

The solution that worked for me was emptying /etc/apt/trusted.gpg.d, running apt-get update, and then manually adding every key it blocked on to the main /etc/apt/trusted.gpg keyring with apt-key adv as described above. Nothing else turned out to be necessary, you can leave the /etc/apt/sources.list.d/* where they are. There's probably a smoother way to consolidate all the keyrings in /etc/apt/trusted.gpg.d/* into the main one using gpg itself, however, that was quicker than writing a script.

I really shouldn't have to do this in the first place.