Well that's the dilemma in a multi-threat environment, isn't it?
Do I fire one round on each target and then go back for clean up if necessary, risking target #1 - even if hit - might still be in the fight and get to me while I'm servicing target #4... or do I double tap targets #1, 2 and 3 risking that the additional time for those three additional shots allows target #4 to get to me?
I know a good double is not much more time than a single, in theory, but I'm not there yet with full house defense loads.
Two to the chest, one to the head (with the 9mm sidearm.)
Double tap without wasting time!