Find duplicate customers in SQL Server

Someone typed the same person in twice. Reports count them as two customers. You want the keys that collide, then the rows.

Which emails appear more than once

SELECT LOWER(LTRIM(RTRIM(email))) AS email, COUNT(*) AS times
FROM Customers
GROUP BY LOWER(LTRIM(RTRIM(email)))
HAVING COUNT(*) > 1
ORDER BY times DESC;

Trim and lower first. Sam@Harbour.co and sam@harbour.co are one person in the real world.

Then the actual rows

SELECT c.customer_id, c.first_name, c.last_name, c.email
FROM Customers c
WHERE LOWER(LTRIM(RTRIM(c.email))) IN (
  SELECT LOWER(LTRIM(RTRIM(email)))
  FROM Customers
  GROUP BY LOWER(LTRIM(RTRIM(email)))
  HAVING COUNT(*) > 1
)
ORDER BY c.email, c.customer_id;