title: Get unique values from array of arrays
body: <p>i have the following array </p>
  
  <pre><code>let arr =   [
    [
      "s1@example.com",
      "s2@example.com"
    ],
    [
      "s1@example.com",
      "s3@example.com"
    ]
  ]
  </code></pre>
  
  <p>i want get the unique values from this array. so im expecting my result to be like this </p>
  
  <pre><code>[
    [
      "s1@example.com",
      "s2@example.com",
      "s3@example.com"
    ]  
  ]
  </code></pre>
  
  <p>i have used array unique function but not able to get the result </p>
  
  <pre><code>var new_array = arr[0].concat(arr[1]);
  var uniques = new_array.unique();
  </code></pre>
  
  <p>this worked if i have two index but how about for multiple index?</p>