The other day I was wondering about words like double and triple, so I decided to look them up. The dou- and tri- parts are pretty obvious, but I had no idea what -ble/-ple meant. It also occurred to me that multiple was probably connected.
According to the OED, the original form of the ending in Latin was -plus, which is also the source of the word plus in English today. Apparently this root goes back to a Proto-Indo-European word meaning "full," though some other sources connect it to a different PIE word meaning "fold," which would make a word like double perfectly cognate with the native English twofold.
By the way, forms like double and treble appear to have come via French, while duple and triple came more directly from Latin.
But the word single is completely unrelated. It comes from singulum, which is a diminutive form of sim, the same root found in simple and (possibly) sincere. Sim comes from the PIE *sem, meaning "one, together." Descendents of that root include the English same and the Greek homo.