A payment retry bug created duplicate orders: same customer, same date, same total. Find each group of duplicates and return customer_id, order_date, total, copies (how many rows are in the group) and keep_id (the lowest order id, the one to keep). Only include groups with more than one row. Sort by customer_id, then order_date.
The shop data has no duplicates, so test against your own rows too.
Schema
customers(id, name, country, signup_date)
orders(id, customer_id, order_date, total)