Internal error: Did not expect <cache file> to already exist

This is the error I'm getting from Wireit:

```
Internal error! Please file a bug at https://github.com/google/wireit/issues/new, mention this message, that you encountered it in wireit version 0.14.9, and give information about your package.json files.
Unknown error thrown: Error: Did not expect /Users/justin/Projects/.../.wireit/74657374/cache/05ee0abc22ae7b2f80e8007ee43757b537ae0f931c959e82826cab8b22ec8f10 to already exist.
```

The error persists unless I delete the .wireit directory. Once I do I can build again, but the error will sometimes pop up again.