for (const [normalPath, metadata] of delta.removed) {
      this.onRemovedFile(normalPath, metadata);
    }
When looking up a module's data, we walk through each eligible platform for
the query. For each platform, we want to check if there are known
duplicates for that name+platform pair. The duplication logic normally
removes elements from the `map` object, but we want to check upfront to be
extra sure. If metadata exists both in the `duplicates` object and the
`map`, this would be a bug.