The motivation is generally to increase their chances of selling a book at an inflated price, by avoiding having their copy listed on the same page as the in-print version or cheaper second hand copies. This is against Amazon rules specifically "Creating a product detail page for a product already in the Amazon catalog is prohibited."
After trying a few different approaches I seem to have hit upon a way to get these duplicate listings removed with a minimum of effort.
1. Open a Seller Account
This is the only inconvenient part. The best avenue for getting listings merged is to open an Amazon seller account. This is free to open and requires no payment or other actions. And as a bonus you get access to the Amazon Seller forums which can be interesting....
2. Get the ISBN or ASIN for the the correct and the duplicate listing
You need to identify the correct and duplicate books by their number, and make sure you don't accidentally get the number for the Kindle version.
3. Request that the Listings be Merged
Go to the Amazon seller report page. You don't need to specify in which way because the title will merge back to your original listing no matter how it is actioned.
- Select Products and Inventory > Product page issue.
- Under What kind of issue are you having? Select Merge or split product page.
- Select Merge and provide the requested information