I have encountered this problem a while ago and it instantly appeared to me as an inceredibly important challange. The problem is in deatil described in "Infinite Ethics", a paper of Nick Bostrom. It is available online and I encourage You to have a look at it.  https://www.nickbostrom.com/ethics/infinite.html 

 In short, citing Nick Bostrom:

"ABSTRACT

Aggregative consequentialism and several other popular moral theories are threatened with paralysis: when coupled with some plausible assumptions, they seem to imply that it is always ethically indifferent what you do.  Modern cosmology teaches that the world might well contain an infinite number of happy and sad people and other candidate value-bearing locations.  Aggregative ethics implies that such a world contains an infinite amount of positive value and an infinite amount of negative value.  You can affect only a finite amount of good or bad.  In standard cardinal arithmetic, an infinite quantity is unchanged by the addition or subtraction of any finite quantity.  So it appears you cannot change the value of the world.  Modifications of aggregationism aimed at resolving the paralysis are only partially effective and cause severe side effects, including problems of “fanaticism”, “distortion”, and erosion of the intuitions that originally motivated the theory.  Is the infinitarian challenge fatal?

1.  The challenge

1.1.  The threat of infinitarian paralysis

When we gaze at the starry sky at night and try to think of humanity from a “cosmic point of view”, we feel small.  Human history, with all its earnest strivings, triumphs, and tragedies can remind us of a colony of ants, laboring frantically to rearrange the needles of their little ephemeral stack.  We brush such late-night rumination aside in our daily life and analytic philosophy.  But, might such seemingly idle reflections hint at something of philosophical significance?  In particular, might they contain an important implication for our moral theorizing?

 If the cosmos is finite, then our own comparative smallness does not necessarily undermine the idea that our conduct matters even from an impersonal perspective.  We might constitute a minute portion of the whole, but that does not detract from our absolute importance.  Suppose there are a hundred thousand other planets with civilizations that had their own holocausts.  This does not alter the fact that the holocaust that humans caused contributed an enormous quantity of suffering to the world, a quantity measured in millions of destroyed lives.  Maybe this is a tiny fraction of the total suffering in the world, but in absolute terms it is unfathomably large.  Aggregative ethics can thus be reconciled with the finite case if we note that, when sizing up the moral significance of our acts, the relevant consideration is not how big a part they constitute of the whole of the doings and goings-on in the universe, but rather what difference they make in absolute terms.

The infinite case is fundamentally different.  Suppose the world contains an infinite number of people and a corresponding infinity of joys and sorrows, preference satisfactions and frustrations, instances of virtue and depravation, and other such local phenomena at least some of which have positive or negative value.  More precisely, suppose that there is some finite value ε such that there exists an infinite number of local phenomena (this could be a subset of e.g. persons, experiences, characters, virtuous acts, lives, relationships, civilizations, or ecosystems) each of which has a value ≥ ε and also an infinite number of local phenomena each of which has a value ≤ (‒ ε).  Call such a world canonically infinite.  Ethical theories that hold that value is aggregative imply that a canonically infinite world contains an infinite quantity of positive value and an infinite quantity of negative value.  This gives rise to a peculiar predicament.  We can do only a finite amount of good or bad.  Yet in cardinal arithmetic, adding or subtracting a finite quantity does not change an infinite quantity.  Every possible act of ours therefore has the same net effect on the total amount of good and bad in a canonically infinite world: none whatsoever.

Aggregative consequentialist theories are threatened by infinitarian paralysis: they seem to imply that if the world is canonically infinite then it is always ethically indifferent what we do.  In particular, they would imply that it is ethically indifferent whether we cause another holocaust or prevent one from occurring.  If any non-contradictory normative implication is a reductio ad absurdum, this one is. 

Is the world canonically infinite or not?  Recent cosmological evidence suggests that the world is probably infinite.[1]  Moreover, if the totality of physical existence is indeed infinite, in the kind of way that modern cosmology suggests it is, then it contains an infinite number of galaxies, stars, and planets.  If there are an infinite number of planets then there is, with probability one, an infinite number of people.[2]  Infinitely many of these people are happy, infinitely many are unhappy.  Likewise for other local properties that are plausible candidates for having value, pertaining to person-states, lives, or entire societies, ecosystems, or civilizations—there are infinitely many democratic states, and infinitely many that are ruled by despots, etc.  It therefore appears likely that the actual world is canonically infinite.

 We do not know for sure that we live in a canonically infinite world.  Contemporary cosmology is in considerable flux, so its conclusions should be regarded as tentative.  But it is definitely not reasonable, in light of the evidence we currently possess, to assume that we do not live in a canonically infinite world.  And that is sufficient for the predicament to arise.  Any ethical theory that fails to cope with this likely empirical contingency must be rejected.  We should not accept an ethical theory which, conditional on our current best scientific guesses about the size and nature of the cosmos, implies that it is ethically indifferent whether we cause or prevent another holocaust."

Many potential solutions were presented in Bostrom's work, yet none of them eventually seems to be the one. Problems of some are solved by other solutions, but they on the other hand cause another, often more abstract problems.  Seriously, read it all https://www.nickbostrom.com/ethics/infinite.html

Under unification we cannot have infinite suffering. 

Two identical experiences, two copies of the same experience in some system can be interpreted as two persons or as just one person, instantiated in two places. It is an important issue in the infinite multiverse, because there is an infinite amount of every state of mind. Duplication is a view that two copies of the exact same state of mind are two people, suffering of two identical copies is two times higher than just one, unification is an interpretation according to which in such a case there would be only one person. 

In the infinite multiverse, if there is only a finite amount of possible (so, under modal realism, realized) states of mind, we cannot have an infinite amount of suffering. 

Nick Bostrom argues toward duplication, yet despite its intuitive appearance, duplication has some problems when it comes to explaining what is going on in some thought experiments, potentially possible to conduct in the future, especially in simulated realities, 

https://www.nickbostrom.com/papers/experience.pdf

On the other side,  Arnold Zuboff is a supporter of Unification. 

Here; https://www.researchgate.net/publication/233329805_One_Self_The_Logic_of_Experience

and here; https://www.researchgate.net/publication/282052756_Time_Self_and_Sleeping_Beauty

I think, If we assume unification, there is no infinitarian paralysis. We could change the chances our future will contain less suffering, and do so with every possible mindstate, minimizing suffering (since to reduce the number of possible states of suffering is impossible under any assumptions, all we can do is to change probabilities of subjective futures)