How to delete duplicate data after a loop cascade drop-down box in a php webpage

How to delete duplicate data in the loop cascade drop-down box on the php webpage. for example, a material has a variety of styles, different thicknesses of the cloth, and different thicknesses of the resin cloth.
Cascade drop-down lists using Javascript in PHP web pages. Use a material as a drop-down box (primary choice). when selecting one of the materials, how to assign the corresponding operator to a drop-down box (derivative Box 1 ), the corresponding resin is assigned to another drop-down box (derivative box 2 ),. my code is as follows. Why does the derivative box contain duplicate data? How can I delete this duplicate data?










Header ("Content-Type: text/html; charset = gb2312 ");
$ Link = mysql_connect ("localhost", "root", "12345670 ")
Or die ("cocould not connect:". mysql_error ());
Mysql_select_db ("test") or die ("cocould not select database ");
Mysql_query ("set names 'gb2312 '");
$ SqlSel = "select * from material ";
$ Result = mysql_query ($ sqlSel) or die ("Query failed:". mysql_error ());

$ Forum_data = array ();
While ($ row = mysql_fetch_array ($ result ))
{
$ Forum_data [] = $ row;
}
// Print_r ($ forum_data );
Mysql_free_result ($ result );


$ SqlSel2 = "select * from core left join prepreg on core. Material = prepreg. Material ";

If (! ($ Result2 = mysql_query ($ sqlSel2 )))
{
Die ('could not query core list ');
}

$ Forum_data2 = array ();
While ($ row2 = mysql_fetch_array ($ result2 ))
{
$ Forum_data2 [] = $ row2;
}

Mysql_free_result ($ result2 );

?>