Muggles' Guide to Harry Potter/Magic/Gemino

Overview
Gemino is a protective curse; cast on an object, it makes that object multiply repeatedly when it is touched.

Extended Description
The verbal component of this spell is based on the word Gemini, which is the English form of the zodiac symbol for "the twins".

According to Griphook, it is apparently fairly common for valuable items in the vaults at Gringotts to be protected with the Gemino and Flagrante curses. The former causes the touched item to produce multiple copies of itself, thus confusing the burglar into taking the valueless copies.

Analysis
Note the similarity to the Geminio charm, which produces a single replica of the object it is cast upon.