"Don't take no chances, two shots in the head a piece!"
When Al Neri takes out Barzini, he shoots the two guys up close (one guy gets two bullets the other just one). Then he shoots Barzini twice in the back from long range.
Now consider this: Vito was shot five times in the back from close range and lived. What if Barzini had made it through alive?
Surely Clemenza would have given Neri the same advice? Also, what would have happened among the five families had Barzini lived?