You export your products and you get an error message like this one:
Couldn't import a duplicated Reference/SKU (XXX-YYY)
The concerned product is a duplicate, that means that the product reference is present at least twice in the database which is wrong.
Example for duplicate:
You must ensure that each product and each combination has a unique reference as shown on the page: validate your catalog structure.
Action: Correct the duplicated product reference.