Return an array of `keys` for which their `value` in `object` are duplicates

duplicates.js

// Return an array of `keys` for which their `value` in `object` are duplicates
function duplicates(object) {
  const keys = Object.keys(object);
  const values = keys.map(k => object[k]);
  const arr = keys.map(key => ({ key, value: object[key] }));
  const _duplicates = new Set();
  let possibleDuplicate;

  for (const v of values) {
    possibleDuplicate = arr.filter(a => a.value === v);
    if (possibleDuplicate && possibleDuplicate.length > 1) {
      possibleDuplicate = possibleDuplicate.forEach(d => _duplicates.add(d.key));
    }
  }

  return Array.from(_duplicates);
}

Example

duplicates({
 x: 1,
 y: 2,
 z: 1,
 xx: 1,
 L: 0,
});

yields

=> [ 'x', 'z', 'xx' ]

With lodash

Array.from(new Set(Object.values(_.groupBy(Object.entries({
 x: 1,
 y: 2,
 z: 1,
 xx: 1,
 L: 0,
}), i => i[1])).filter(a => a.length > 1).flat().map(a => a[0])))