# [Javascript (ES6)](https://developer.mozilla.org/bm/docs/Web/JavaScript), 34 bytes

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

Try it: 

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

<!-- language: lang-js -->

    function getDupes(a) {
        return a.sort().filter((v,i)=>v===a[i+1])
    }

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

<!-- end snippet -->