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: