Duplicate keys should error

[EvanHahn]

This should be invalid:

csp({
  directives: {
    'script-src': ['example.com'],
    scriptSrc: ['example.com']
  }
})

This is a breaking change, so we'll need to do it in Helmet 4.

[knoxcard]

I agree, this is easy code to write and I found myself doing this a bunch of times.

[EvanHahn]

This has been addressed in helmet@4 and helmet-csp@3.0.0.

[EvanHahn]

This has been addressed in helmet@4 and helmet-csp@3.0.0.

I'm going to be archiving this repository soon and moving everything to https://github.com/helmetjs/helmet/, so feel free to open an issue there if you run into any problems.