Your error is because you have:
JOIN user ON article.author_id = user.id
LEFT JOIN user ON article.modified_by = user.id
You have two instances of the same table, but the database can't determine which is which. To fix this, you need to use table aliases:
JOIN USER u ON article.author_id = u.id
LEFT JOIN USER u2 ON article.modified_by = u2.id
It's good habit to always alias your tables, unless you like writing the full table name all the time when you don't have situations like these.
The next issues to address will be:
SELECT article.* , section.title, category.title, user.name, user.name
1) Never use SELECT * - always spell out the columns you want, even if it is the entire table. Read this SO Question to understand why.
2) You'll get ambiguous column errors relating to the user.name columns because again, the database can't tell which table instance to pull data from. Using table aliases fixes the issue:
SELECT article.* , section.title, category.title, u.name, u2.name