What does this error mean?

duplicate key value violates unique constraint "product_attribute_value_product_product_rel_att_id_prod_id_key" DETAIL: Key (att_id, prod_id)

This is the csv file I exported. When I try to reimport it I get this error. When I delete the second row it works. 

"id","name","type","product_variant_ids/attribute_line_ids/value_ids","product_variant_ids/attribute_line_ids/attribute_id","product_variant_ids/default_code"
"__export__.product_template_3","ProductName","Stockable Product","Att1,Att2","ProductAtt","12345"
,,,"Att1,Att2","ProductAtt","12346"