That's what you would initially think, but a clever adversary can produce arbitrarily many identities, so even if results agree, there is a non-zero chance that all of the agreeing nodes are compromised. Even if you begin blacklisting nodes, an attacker can simply spoof new ones.

So then you need a proof of work scheme to defend against sybil attacks, but even then an attacker could choose to only produce incorrect output when they detect that a quorum group is made entirely of their compromised nodes, and otherwise produce correct output. That would once again render blacklisting ineffective.

I'm not saying it's impossible, but securing this sort of browser-based grid computing scheme is a nontrivial task. And that's coming from someone who is very fond of this technology.

Use homomorphic encryption to ensure attackers can't tell when they get the same thing twice.

IPs are not so cheap, anyway. If you have enough legitimate traffic it would be tough to spoof enough.

IPs are very cheap. You aren't renting them for a month. You are using them for a few requests.

You're thinking in IPv4.

Most traffic will be IPv4, if you only use them for work you'll do ok

How does Steem and Nano solve this? They much have the same problems.

Neither Steem nor Nano seem to be a supercomputer with arbitrary computation.

I'm not very familiar with either of them, but Steem seems to be a non-centralized database of reddit-like posts. This doesn't seem to be offloading arbitrary computations to others like OP wants to do.

Nano also doesn't seem to be offloading arbitrary computations to others.

Ethereum does allow arbitrary computations, but it is not offloading, it is requiring every node to rerun every computation, which is obviously much less efficient than a single centralized computer doing the computation.