I have the book in front of me. Unless there is an errata I don't know about (I have to PHB Errta updates and neither say anything about this) it Repelling blast is written "When you hit a creature" so each hit, knocks its target back 10ft.
Yup, I agree that the RAW is clear. When thinking about balance I have a number of things on my mind. One thing that is
not on my mind is the power of a mechanic: strong mechanics are fine. Rather it is how that mechanic is, in play?
Does it warp encounters around it? So not just - is it powerful - but, do I as DM usually need to adjust the encounter or behaviour of creatures in the encounter to deal with this one mechanic, otherwise the encounter becomes tactically weird or uninteresting?
Does it invalidate other strategies? Essentially, do all players choose this because it's so good that other choices are not worth taking. This is actually quite a tricky question to apply, because there are also mechanics that are foundational to a class - they're how the class operates - and of course all players take those. I'm sure people will have different views on this, but for me
agonizing is foundational, while
repelling invalidates other strategies. Part of the "why" is that
repelling often invalidates other class' strategies!
Repelling is also something of a one trick - does everything all by itself - a good mechanic plays well with other strategies.
When I (experimentally) change it, do players still consistently take it? This for me is a very useful test. If I tune something, and players still want that thing, there's a really good chance it was over-tuned to begin with.
Repelling even once per target is in that bag. Another way of putting this is - does it need to be that good? I think the invocations released with Xanathar's are very suggestive that it does not: that it was an oversight.
Have I enough experience with this in play, to feel confident? This is important. Sometimes, there is a counter or natural balance to something, and you just haven't played enough to know. Some things -
magic stones is an example - are good at certain levels, but stop being good later on.
Repelling is good at every level it is available. In fact, I think it gets better at higher levels because... no save.
Does the game design take proper account of this strategy? Some mechanics are very strong, but then the game design includes counters to them.
Repelling allows a warlock at any level that has it, to push monsters regardless of their CR. Our warlocks in OOTA could push Demogorgon with it. The game design just didn't seem to properly account for the strategy.
So while the RAW is clear, I wouldn't play it that way. Or at the very least, I'd suggest trying an alternative. Is it at the top of the list of things to fix? Probably not, because not all parties will even contain a warlock. If, like mine, your party ends up with two of them... then you might prioritise it higher!