Intenta agrupando:
Código PHP:
$sql_in=execute("SELECT L.*, P.* FROM userlist as L right join post as P
on P.userid = L.relationid WHERE (L.userid= '".$_SESSION['userid_s']."'
AND L.friend = 'yes') OR P.userid='".$_SESSION['userid_s']."'
GROUP BY P.userid
ORDER BY P.postid DESC LIMIT 5");
Saludos