The Gemino Curse or Doubling Charm (incantation: Geminio) is a spell used to duplicate an object.
The Doubling Charm was invented by a pair of reclusive twin witches, Helixa and Syna Hyslop, who used it to create duplicates of every item inside the mansion in which they lived together their whole lives. After their deaths, their relatives finally learned of this practice, discovering two sets of hand-written instructions for the spell, one left by each twin.
There was much debate over whether a copy created with the Gemino Curse held the same value as the original. The two items were impossible to tell apart at first due to their identicalness. However, over time, the copy tended to rot or tarnish more quickly than the original, making it possible to identify eventually. An odd characteristic of the spell, for which a solution was never found, was that only the original caster could stop it. If for some reason the caster was interrupted, the object would continue multiplying for hours or even days, stopping only when the copies began to break down. This indefinite replication property could apparently be harnessed as a security measure.
The treasure in the Lestrange Vault at Gringotts Wizarding Bank was bewitched with this spell so that it would begin multiplying out of control when touched by an intruder, thus quickly filling up the space, and trapping or crushing the trespassers.