get all posts with their associative tags without duplicates
You didn't provide DDL and sample data so we can't test anything.
Also, it is not clear what is the join predicate, but I think you are looking for GROUP_CONCAT, something along the lines of (Pseudo code):

SELECT P.Title, GROUP_CONCAT(T.Tag SEPARATOR ',') AS Concatenated_Tags
FROM Post P INNER JOIN Tag T ...
GROUP BY P.Title;


HTH