select a,b,count(*) from my_table where ... group by a,b;

$max= 0 ;
while (t=mysql_fetch_array($result)) ($t>$max ? $max=$t : $max)

SELECT a,b,count(*) AS cnt FROM my_table WHERE ... GRUOP BY a,b ORDER BY CNT DESC LIMIT  5 

$max = select count(*) AS CNT from my_table where ... group by a,b ORDER BY CNT DESC LIMIT  1  

$query1 =  "select a,b from my_table where user_id='$user[user_id]' and duty='0'" ;
$result1 = mysql_query($query1,$link)or die( "Unable to query" );
while ($row = mysql_fetch_array($result1))...

user_id!='$user[user_id]' & duty='0' & a= $row[ 0 ] & b=$row[ 1 ]