How to delete from select in MySQL?

SELECT (sub)queries return result sets. So you need to use IN, not = in your WHERE clause.

Additionally, as shown in this answer you cannot modify the same table from a subquery within the same query. However, you can either SELECT then DELETE in separate queries, or nest another subquery and alias the inner subquery result (looks rather hacky, though):

DELETE FROM posts WHERE id IN (
    SELECT * FROM (
        SELECT id FROM posts GROUP BY id HAVING ( COUNT(id) > 1 )
    ) AS p
)

Or use joins as suggested by Mchl.

DELETE 
  p1
  FROM posts AS p1 
CROSS JOIN (
  SELECT ID FROM posts GROUP BY id HAVING COUNT(id) > 1
) AS p2
USING (id)

you can use inner join :

DELETE 
    ps 
FROM 
    posts ps INNER JOIN 
         (SELECT 
           distinct id 
         FROM 
             posts 
         GROUP BY id  
      HAVING COUNT(id) > 1 ) dubids on dubids.id = ps.id