wrong behavior when flatten-duplicate-arrays and duplicate-arguments-array are enabled

[bcoe]

@nexdrew pointed out this edge-case:

when you enable both flatten-duplicate-arrays and duplicate-arguments-array are enabled

{
  "yargs": {
    "flatten-duplicate-arrays": false,
    "duplicate-arguments-array": false
  }
}

Running:

node test.js -x 1 2 3 -x 2 3 4

results in:

{ _: [],
  x: [ [ '1', '2', '3' ], [ '2', '3', '4' ] ],
  '$0': 'test.js' }

It seems like it would be more consistent with this new API for the result to be x: ['2', '3', '4']

CC: @laggingreflex if you're looking for your next contribution to yargs, would love help adding a test and fix for this.