I would imagine that the goal of the problem is to see how you think through a problem to identify a root cause, and then implement solutions considering trade-offs.
Understand the problem first to get to the root cause:
Are these products being returned in a particular category i.e apparel versus watches? This might give some clues about how the product descriptions or SKUs are being listed on the website today.
Are these FBA or are these shipped directly by merchants to customers? How much can Amazon control in this process? Is there an error in how the products are being stocked in the warehouse or is there a problem with how the seller has separated SKUs?
Is this problem pervasive across sellers in a single category or multiple sellers in multiple categories?
Are these products being shipped from a new fulfillment center ? Was there a turnover in staff recently in these fulfillment centers?
Is this a new product category for Amazon? Are the product descriptions accurate?
1. If the problem is specific to a category i..e tee shirts, see whether the product detail pages for these items identify the right item to ship based on Color (blue, black, white) , Size (S,M,L) and any other factors that can multiply the inventory.
2. If the problem is specific to a seller, investigate Do items have duplicate bar codes even though they are completely different? check if the seller is new or requires better quality control.
3. If the problem is with a specific fulfillment center and not a specific type of product or seller, then identify where in the FC the problem occurs.
4. Is the seller shipping on their own? Do they need to be shut down for breaching customer trust?