arr.reduce((acc, cur, idx) => {
  if (typeof acc.dic[cur] === 'undefined') {
    acc.dic[cur] = idx;
  } else {
    acc.repeated.push(idx);
  }
  return acc;
}, {dic: {}, repeated: []});

const counted = arr.reduce((acc, cur, idx) => {
    if (typeof acc[cur] === 'undefined') {
    acc[cur] = [idx]
    } else {
    acc[cur].push(idx);
    }
  return acc;
}, {});

Object.values(counted)
      .filter(item => item.length > 1)
      .reduce((acc,cur) => acc.concat(cur))