Dedupe a signup list — the fast version

 1function dedupeByEmail(users) {
 2  const seen = new Set();
 3  const out = [];
 4  for (const u of users) {
 5    if (!seen.has(u.email)) {
 6      seen.add(u.email);
 7      out.push(u);
 8    }
 9  }
10  return out;
11}