multiple patterns matching the same file will break build

assuming a preprocessors config like this:

```
preprocessors: {
'app/js/app-module.js': [ 'coverage' ],
'app/js/*.js': [ 'coverage' ]
},
```

`app-module.js` will get instrumented and the output from that will get instrumented again, totally breaking things.

Obviously, the easy answer is "Don't do that", but:
1. I have my reasons for wanting to do it this way.
2. Double processing a file in this way will always be a mistake, so why not just avoid it.
3. Most importantly: the failure message gives no indication of what broke the build. I got `caught TypeError: Cannot read property '2' of undefined`, not super helpful in locating the problem.

I'm happy to work on a PR if you are willing to consider it.