Does Repelling Blast work once per spell or once per beam?

Warlocks can take the following invocation:

Repelling Blast
Prerequisite: eldritch blast cantrip

When you hit a creature with eldritch blast, you can push the creature up to 10 feet away from you in a straight line.

This could be read as "when you hit a creature with your eldritch blast cantrip", i.e. once per turn regardless of beams/damage, or "whenever a creature gets hit with one of your eldritch blasts", which might happen more than once per turn. Which is correct? Can a Level 5 Warlock push a single goblin back 20′ per round if they hit twice? How about two goblins for 10′ each?