const duplicates = Array
.from(arr.reduce((acc, n) => acc.set(n.id, acc.has(n.id)), new Map))
.reduce((acc, n) => (n[1] && acc.push(n[0]), acc), []);
// или
const duplicates = arr.reduce((acc, { id }) => (
(acc[1][id] = -~acc[1][id]) === 2 && acc[0].push(id),
acc
), [ [], {} ])[0];
// или
const duplicates = [...arr
.reduce((acc, n) => (acc[+acc[0].has(n.id)].add(n.id), acc), [ new Set, new Set ])
.pop()
];
// или
const duplicates = arr
.map(n => n.id)
.filter((n, i, a) => i !== a.indexOf(n))
.filter((n, i, a) => i === a.indexOf(n));
// или
const duplicates = arr.reduce((acc, { id: n }, i, a) => (
!acc.includes(n) && i !== a.findIndex(m => m.id === n) && acc.push(n),
acc
), []);