An easy way to find duplicates in an array

I was fiddling with my blog. Here is a simple puzzle solution which was finding a duplicate in an array. Array has only one duplicate in it. An easy solution is to use indexOf and lastIndexOf in Javascript.

Here is my findDuplicates function:

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function findDuplicate(array){
  console.log(array.length);
  for (var i = 0; i < array.length; i++) {
    if (array.indexOf(array[i]) != array.lastIndexOf(array[i]))
      console.log(array[i]);
  }
}