Find Duplicate Element in JavaScript Array

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function uniq(l) {
  return l.filter(function (e, i) { return l.indexOf(e) === i; });
}
function findDup(l) {
  return uniq(l.filter(function (e, i) { return l.indexOf(e) !== i; }));
}

Example:

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> findDup([1, 2, 3, 4, 2, 5, 1, 2]);
< [2, 1]