There are many ways to de-duplicate an array. Here are some of the simplest ways

1. Use THE ES6 Set non-repetitive interface

function unique(arr) {
    return Array.from(new Set(arr));
}
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2. Using the indexOf

function unique(arr) {
   let result = []
   for(let index = 0; index < arr.length; index++) {
     let item = arr[index]
     result.indexOf(item) === -1 && (result.push(item))
   }
   return result;
}
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3. Double-layer traversal

function unique(arr) {
  for(let i = 0; i < arr.length-1; i++) {
    for(let j = i + 1; j < arr.length; j++) {
      if (arr[i] === arr[j]) {
        arr.splice(j, 1)        j--
      }
    }
  }
}
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