So I've been digging on this for a few minutes...it turns out that there _are_ duplicate entries in one of the packs. The offending pack is `pack-f9b548a89247792cb59fb86eedf996b32df5cb30.pack`, which does _not_ have object that triggers the problem. It's just that during the search we find out that we don't have it by falling amidst a set of duplicates (of which there three!).

I'm really not sure why this happened; pack-objects has always removed duplicates. That pack is in the actual fork, not the network repo, which means it almost certainly was a pack somebody sent us. So either somebody sent us crap, or `index-pack --fix-thin` can induce duplicates (I haven't checked yet).