# [Javascript (ES6)](https://developer.mozilla.org/bm/docs/Web/JavaScript), 41 bytes

    a=(a)=>a.sort().filter((v,i)=>v===a[i+1])

### Expanded: 

    function getDuplicates(arr) {
        // This makes use of the fact that sort() mutates the original array
        arr.sort().filter((value, index) => value === arr[index + 1]);
    }


Try it: 

<!-- begin snippet: js hide: false console: true babel: false -->

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    a=(a)=>a.sort().filter((v,i)=>v===a[i+1])


    console.log(a([1,2,3]));
    console.log(a([1,2,3,2,1]));
    console.log(a([1.0, 2, 2.0, 1.0]));
    console.log(a([1, '1', '1.0']));
    console.log(a([null, null, false, false, true, true]));

<!-- end snippet -->