username ikisindede aynı olduğu için tekrarlaması normal.
Group by yada distinc ile dene bakalım.

SELECT p.id as 'pollsID',c.id as 'contentID', c.title as 'contentTitle',p.title as 'pollsTitle' FROM forum_content as c INNER JOIN forum_polls  as p ON c.username=p.username WHERE c.username='{$user->username}' GROUP BY p.id,c.id

Olmazsa şunu dene.

SELECT content.*,polls.* FROM (c.id as 'contentID', c.title as 'contentTitle',c.username FROM forum_content as c WHERE c.username='{$user->username}') as content INNER JOIN (SELECT p.id as 'pollsID',p.title as 'pollsTitle',p.username FROM forum_polls as p WHERE p.username='{$user->username}') as polls ON content.username=polls.username