Since I don't know exactly how your DB is structure, you could go with your second idea of storing the output into an array. Then compare what's in the array with your second output and don't show if it's in the array.

So create an array like so:

$first_list = array();


Then in your first query/output.

$cat_name = $rs['cat_name'];

Dump the names into that array.

$first_list[] = $cat_name;


Then in your second query, you can compare the value in the $first_list array and just not output anything that matches...