how to make duplicate array with existing item in javascript code example

Example 1: javascript duplicate an array

let arr =["a","b","c"];
// ES6 way
const duplicate = [...arr];

// older method
const duplicate = Array.from(arr);

Example 2: duplicate numbers in an array javascript

[1, 1, 2, 2, 3].filter((element, index, array) => array.indexOf(element) !== index) // [1, 2]