No, don't do that. Players will not have multiple pollution combinators in the same chunk precisely because it is redundant.
Don't trust anybody: not modders, and certainly not players! :-D
You skipped the the real point of my argument. It doesn't matter if you do multiple CHEAP api calls, so time spent avoiding them is time wasted by CPU and the coder. So you are over engineering a solution to a problem that probably doesn't exist even when players do place several pollution combinators in each chunk, where you aren't sure the the "solution" that your code introduces isn't slower than just running the API calls twice. Or did you measure this? If you have 1024 pollution combinators in each chunk then sure your solution might be faster. But no-one does that and it is not worth optimising for. Also, pollution changes all the time, so your "has pollution updated" check is just more overhead that you think speeds it up but I would guess actually slows it down just by storing the old value and doing lookups and comparisons and branching.
And as the potential user of the mod who knows pollution is by chunks and that pollution is updated all the time (as in every time pollution simulation is updated all chunks with pollution will adjust slightly due to spreading and absorption and will not stay exactly the same) I will never benefit from your suggested/implemented "optimizations". My map doesn't have absurd 0 spread and absorption map settings with basically no active polluters at all so pollution values will always update. I won't place 1024 pollution combinators in each chunk. For me, a sane user, your changes are both guaranteed to slow me down (0 checks are faster than some checks) and your code will never be able to reduce updates to combinators or api calls for pollution checking because a sane factory will have minimum api calls naturally. And the necessary data structures you build are pure bloat multiplying the data that is already stored by Factorio. And you are doing all these (admittedly probably very small) slowdowns for the 99.9% of users in the hope that you might reduce the performance hit (that you don't even know if it exists) for the 0.1% of insane users that try to slow down their own factories! Please, I beg you, reconsider and contemplate if you are wasting your time and making your mod unmaintainable with needless complications to your code.
I can trust myself to not do stupid stuff.
What I might have is a pollution combinator in each chunk (realistically probably much less), potentially thousands or tens of thousands of pollution combinators. And pollution takes minutes to really spread and I will only read them once every few seconds and don't even care if the values are a minute old for the factory I have in mind. Not iterating over 10k entities every tick and instead processing 1 per tick can speed it up by 10k times for my usecase. I will probably not have that many pollution combinators and reducing updates to just 1 per tick is probably a bit extreme, but still this is a simple and effective optimization that scales and will be more helpful to extreme pollution combinator build spammers and not affect people with a reasonable amount of pollution combinators at all.