allowMultipleEquivalentWritesToOneLocation

It's possible that multiple codepaths from multiple tests can end up writing a single snapshot to a single location. If all these codepaths are writing the same value, it's fine. But it's a bit of a problem waiting to happen, because if they start writing different values, we'll have a "snapshot error" even within a single invocation, so it can't be resolved by updating the snapshot. By default we let this happen and give a nice error message if it goes wrong, but you can disallow it in the first place if you want.