need help building a query to detect duplicates on multiple columns
You can accomplish this with grouping on your store_id and value in a CTE or subquery to get the dupes and then re-joining the dupes to your  catalog_product_entity_varchar table like so:
WITH CTE_Dupes AS
(
     SELECT store_id, value
     FROM catalog_product_entity_varchar
     WHERE attribute_id = 86
     GROUP BY store_id, value
     HAVING COUNT(1) &gt; 1
)

SELECT C.entity_id, C.store_id, C.value
FROM catalog_product_entity_varchar C
INNER JOIN CTE_Dupes D
    ON C.store_id = D.store_id
    AND C.value = D.value
WHERE C.attribute_id = 86