Checking for duplicate entries for Column A - Table A based on user_id from Table B
Do a GROUP BY aggregation and see if the aggregated count is greater than 1

SELECT B.Author_id,A.Skill_id,COUNT(1) SkillEntryCount
FROM tableB B INNER JOIN tableA A USING (Entry_id)
GROUP BY B.Author_id,A.Skill_id
HAVING COUNT(1) &gt; 1;


or collect all entry_id values for each (Author_id,Skill_id) and see it is has commas

SET group_concat_max_len = 1048576;
SELECT * FROM
(
  SELECT B.Author_id,A.Skill_id,GROUP_CONCAT(A.Entry_id) EntryIDs
  FROM tableB B INNER JOIN tableA A USING (Entry_id)
  GROUP BY B.Author_id,A.Skill_id
) AA WHERE LOCATE(',',EntryIDs) &gt; 0;