MySQL & PHP - Not unique table/alias

Question

I get the following error listed below and was wondering how do I fix this problem.

Not unique table/alias: \'grades\'

Here is the code I think

Answer 1

I thin that in the $sql2 query the second table isn't grades but article_grades. so it will be:

"SELECT COUNT(*) 
         FROM grades 
         JOIN articles_grades ON grades.id = articles_grades.grade_id
         WHERE articles_grades.users_articles_id = '$page'"