Fixes redundant columns compiled for join variables in queries

[netj]

?- articles(id1, content), articles(id2, content).

It was producing an extra column for every join variable like:

SELECT R0.article_id AS "id1", R0.text AS "content", R1.article_id AS "id2", R0.text AS "content"
FROM articles R0, articles R1
        WHERE R1.text = R0.text