Sometimes in VATS my single shot pistol will fire 2 bullets instead of 1. How can I fire multiple shots? It seemingly only does it at random. I think it might be a glitch, since this game is very glitchy. Is it a glitch?

it is not a glitch, some guns when you shoot it will shoot more than one bullet for example anything semiautomatic will shoot more than one bullet

or you could just hit the RT button more than once while in vats, and it will shoot multiple times at the target.