Comment by Smaug123

Ish. This is covered in section 6.4.1 of Eelco's thesis (https://edolstra.github.io/pubs/phd-thesis.pdf). It all becomes much simpler if evaluating a build many times can only ever result in one output, but the Nix content-addressed model does permit multiple outputs. In such cases, the system just has to choose a canonical output and use that one, rewriting hashes as necessary to canonicalise inputs which are non-canonical.