Double clutching is a term that applies to reptiles who generally only have one clutch a year.
Leopard Geckos mate at the beginning of the season and then lay multiple pairs of eggs, spaced out through the summer, while retaining sperm from that mating. The amount of clutches they can have increases for the first few years of their life and then gradually decreases until they're past their "prime."
Snakes on the other hand (a good example would be cornsnakes) mate and then have one clutch of eggs directly after that mating. They can have several to many eggs and are not limited to two at a time, the way leos are. The instinct to mate is generally brought on by warmer temps so if a pair are mated early and produce eggs early, they can be rebred before the warm season has passed. Having a second clutch of eggs in the same warm season is called double clutching.