I'm sure every DM has had this happen: one PC is trying to find something cool on a dead monster's body. The player rolls poorly, and instead of accepting the paltry 2 copper pieces, the PC calls all the party members over to also try looting the corpse. Invariably, a different player rolls well, and everyone walks away satisfied that everything's been taken down to the poor dead monster's last holey sock.
Another example: One PC is trying to find some useful information in a library. There're no guards, no reason anyone would be bothered, and no time restraints to stress about. The player rolls poorly, so all the other PCs try, too.
I'd describe this as well-intentioned metagaming. The players are afraid of losing out on content, this despite multiple sessions of me making it clear that I don't structure my games to penalize gameplay that way.
These are situations that require a roll to determine the degree of success. (At least I think so; perhaps I'm wrong here?) Failure could set back the PCs a cool item or a bit of information that would have helped a little, but these minor failures aren't game-enders or anything. As a DM I can't see a reason to say "No" and disallow the practice, but it feels vaguely like cheating to let the players roll multiple times to get better results.
How do I prevent players ganging up on skill checks after they've seen someone roll poorly?