Rewatch allows two different package specs with the same suffix

[cknitt]

This works fine in rewatch - for an input file A.res, both A.cjs and A.mjs are produced:

{
  "name": "x",
  "sources": ".",
  "package-specs": [
    {
      "module": "commonjs",
      "in-source": true,
      "suffix": ".cjs"
    },
    {
      "module": "esmodule",
      "in-source": true,
      "suffix": ".mjs"
    }
  ]
}

However, it is possible to specify the same suffix twice (or omit it for both, defaulting to .js), leading to undefined/unexpected behavior.