Keep The Newest Signup

signups:

id	email	created_at
1	a@x	2026-01-02
2	b@x	2026-01-03
3	a@x	2026-01-05
4	c@x	2026-01-06
5	a@x	2026-01-07

WITH ranked AS (
  SELECT id, email,
         row_number() OVER (PARTITION BY email
                            ORDER BY created_at DESC, id DESC) AS rn
  FROM signups
)
SELECT id FROM ranked WHERE rn = 1 ORDER BY id;

Which ids come back?

2, 4, 5

1, 2, 4

1, 2, 3, 4, 5