Detecting a duplicate element in array

const array = [1,2,3,4,4];

function findDuplicate(inArr) {
  inArr.reduce((sum, entity, idx) => {
    sum[entity] = true;
    if (Object.keys(sum).length != idx+1) {
      throw `Duplicate found at index ${idx} in value ${entity}`;
    }
    return sum;
  }, {});
}

findDuplicate(array);