how 'not to remove' duplicates while creating an object in es6 code example

Example 1: remove duplicate array es6

let a = [10,20,30,10,30];
let b = a.filter((item,index) => a.indexOf(item) === index);
console.log(b);

Example 2: javascript remove uniques from array

function getNotUnique(array) {
    var map = new Map();
    array.forEach(a => map.set(a, (map.get(a) || 0) + 1));
    return array.filter(a => map.get(a) > 1);
}

console.log(getNotUnique([1, 2, 2, 4, 4])); //[2, 2, 4, 4]
console.log(getNotUnique([1, 2, 3] )); //[]