You export your products and you get an error message like this one:
Product "X/Y" - Duplicate EAN Code
The concerned product contains a duplicate EAN, that means the product EAN code exists two or more times in the database, which is wrong.
Common example of duplicate EAN code:
You must ensure that each product and each combination has a unique EAN code as shown on the page: validate your catalog structure.
You can then find a product displayed in this list of identifiers, through its Product ID by searching in your catalog, for example: