Filter unique or duplicate values from an array

unique.ts

function onlyUnique<T>(value: T, index: number, self: T[]) {
  return self.indexOf(value) === index;
}

function onlyNonUnique<T>(value: T, index: number, self: T[]) {
  return self.indexOf(value) !== self.lastIndexOf(value);
}

const data = [];

const uniques = data.map(l => l.insertId).filter(onlyUnique);
const duplicates = data.map(l => l.insertId).filter(onlyNonUnique).filter(onlyUnique);

This has O(n²) complexity