A pivot via conditional aggregation gives wrong (doubled) numbers. The query is below. What is the most likely cause of the inflated values?

SELECT p.product_id, SUM(CASE WHEN s.region = 'East' THEN s.amount END) AS east, SUM(CASE WHEN s.region = 'West' THEN s.amount END) AS west FROM products p JOIN sales s ON s.product_id = p.product_id JOIN shipments sh ON sh.product_id = p.product_id GROUP BY p.product_id;