Duplicate '.js' extension breaks coverage report

[doque]

Like discussed in #235, adding the .js extension (either via command line or package.json) breaks the test coverage report, as it results in duplicate array elements.

I've set up a git repository to reproduce this bug here: https://github.com/doque/break_coverage.
You can switch between the branches to see that it works when removing the '.js' extension (or if you use my fork), see the README.

I realize this error can be attributed to the user, but feel like this should be avoided, which is why I've opened #237 that fixes this bug.

[bcoe]

@doque give this a spin:

npm i nyc@next

If it's doing the trick I'll promote the candidate release to latest.

[doque]

Works like a charm: https://github.com/doque/break_coverage

Thanks!