Someone said similar elsewhere, but the problem is the "meta code" that does the comparison needs to be hardened as well.

Yes, triple redundancy is nice, but then the problem becomes how to make the code that checks the code also tipple redundant and checked without creating another layer to deal with.

Most software that does this simply tries to make the test code as small and reliable as possible. But it's an interesting challenge to make the test code somehow self checking with the same properties.

Well, physically, you really need three independent machines. Which means three copies of the file, three processes, three verifications, and so on, and a way for those processes to communicate with each other and "vote". Then you run into the problem of definition - is a cluster a quine? Note that most of the "primordial bit flip" problems go away if you make N >> 3 and design each machine to "fail fast".

> Isn't the solution to simply duplicate the code and compare it before execution? Do it 3 times and use the majority

Yes, voting is a common way to deal with radiation or critical software. There are of course also complications about the voting software itself needing to also be redundant. Some can be done with hardware, or you can just radiation harden parts of the system (reducing overall cost).

This is definitely one of those problems where at a quick glance it seems like there's a few simple solutions that can resolve 90% of issues but when you dig into the weeds you find out that 20% dictates most of the outcomes and there's at most a 10% overlap with your quick/obvious solutions.