hifProg

const duplicates = arr.filter((n, i, a) => a.filter(m => m[0] === n[0]).length > 1);

const duplicates = Object
  .values(arr.reduce((acc, n) => ((acc[n[0]] ??= []).push(n), acc), {}))
  .flatMap(n => n.length > 1 ? n : []);

const duplicates = Array
  .from(arr
    .reduce((acc, n) => (acc.get(n[0]).push(n), acc), new Map(arr.map(n => [ n[0], [] ])))
    .values())
  .reduce((acc, n) => (n.length > 1 && acc.push(...n), acc), []);

const duplicates = arr.filter(function(n) {
  return this.get(n[0]);
}, arr.reduce((acc, [ n ]) => acc.set(n, acc.has(n)), new Map));