function duplicates(arr) { return arr.reduce((prev, current, index, arrs) => { if(arrs.lastIndexOf(current) !== index && !prev.includes(current)) { prev.push(current); } return prev; }, []); }
function duplicates(arr) { return arr.reduce((prev, current, index, arrs) => { if(arrs.lastIndexOf(current) !== index && !prev.includes(current)) { prev.push(current); } return prev; }, []); }