I'm new to Prestashop, so my question may seem weird.
So I learned how to add products with CSV files and how to update them.
Just to check I update the products by Forcing ID Checks and User Reference as Primary Key.
Let's say I have an existing Database and CSV files containing new products and some old ones (Which I don't know their IDs in the database). How to add and update products by using the same CSV File? If I disable using reference key and forcing ID checks, I m going to have duplicates with different prices.
As you have posted the same question in Prestashop forums, I have replied there Prestashop forum link,
Quoting for future readers:
If you have access to the backend, which you must have in order to import a CSV simply go to Catalog->Products, get the product ids you need to import and use them in CSV product id column.