dedupMerge

data.js

const people = [
  {
    department: "Directing",
    id: 20561,
    job: "Director",
    name: "James Gray"
  },
  {
    department: "Writing",
    id: 20561,
    job: "Screenplay",
    name: "James Gray"
  },
  {
    department: "Writing",
    id: 1750900,
    job: "Screenplay",
    name: "Ethan Gross"
  },
  {
    department: "Writing",
    id: 20561,
    job: "Dresser",
    name: "James Gray"
  }
];

dedupMerge.js

function dedupMerge(items, map) {
  return Object.values(people.reduce((result, item) => {
    result[item.id] = !result[item.id] ? item : map(result[item.id], item);
    
    return result;
  }, {}));
}