288. Analyzing Sequential Purchase Behavior(https://sqlpad.io/questions/228/analyzing-sequential-purchase-behavior/)

For this question, why are there duplicate values in the results. In the solution code, the following output results: 

Foe many customers such as Ada Lovelace, Bill Gates, Margaret Hamilton, Steve Balmer, and Tim-Berners Lee, there is a duplicate percentage increase for these customers. 

I would like to know why this exists when this is occuring for the same sale.

Additionally, when calculating the purchase for this question, shouldn't it be price * quantity instead of just using price?

The following was my code for the question:

WITH sales_with_lag AS (
    SELECT 
        s.customer_id,
        s.sale_id,
        s.product_id,
        s.sale_date,
        s.quantity,
        s.quantity * p.price AS sale_value,
        LAG(s.quantity * p.price) OVER (PARTITION BY s.customer_id ORDER BY s.sale_date) AS prev_sale_value,
        LAG(s.sale_id) OVER (PARTITION BY s.customer_id ORDER BY s.sale_date) AS prev_sale_id,
        LAG(s.sale_date) OVER (PARTITION BY s.customer_id ORDER BY s.sale_date) AS prev_sale_date,
        LAG(s.product_id) OVER (PARTITION BY s.customer_id ORDER BY s.sale_date) AS prev_product_id,
        ((s.quantity * p.price) - LAG(s.quantity * p.price) OVER (PARTITION BY s.customer_id ORDER BY s.sale_date)) / 
        LAG(s.quantity * p.price) OVER (PARTITION BY s.customer_id ORDER BY s.sale_date) * 100 AS percentage_increase
    FROM msft_sales s
    JOIN msft_products p ON s.product_id = p.product_id
)
SELECT 
    c.name AS customer_name,
    pp.product_name AS lower_value_product_name,
    cp.product_name AS higher_value_product_name,
    swl.prev_sale_date AS lower_value_sale_date,
    swl.sale_date AS higher_value_sale_date,
    swl.percentage_increase
FROM sales_with_lag swl
JOIN msft_customers c ON swl.customer_id = c.customer_id
JOIN msft_products cp ON swl.product_id = cp.product_id
JOIN msft_products pp ON swl.prev_product_id = pp.product_id
WHERE swl.percentage_increase >= 50
ORDER BY c.name, swl.sale_date;

Please let me know why the solution is the way that it is.