Sql query triggered twice

Try Applying These Changes

1. Add this conditional before the // first check if data exists with select query:

   // Do nothing else if the search term is empty.
   if ( empty( $search_term ) ) {
       return $where;
   }
   

   Update: You may also want to check if it’s the main query and if not, exit the function:

   if ( empty( $search_term ) ||        // missing search term
       ! $wp_query->is_main_query() ) { // or not the main query
       return $where;
   }
   

2. I would not do SELECT * (select all) records use $wpdb->get_results() just to check if an existing entry for a search term exists in the database or not. Secondly, you should’ve used $wpdb->prepare() instead of the TERM = '".$search_term."'. So I’d replace this:

   $datum = $wpdb->get_results("SELECT * FROM search_product WHERE TERM = '".$search_term."'");
   if($wpdb->num_rows > 0) {
   

   with this:

   $sql = $wpdb->prepare( "SELECT TERM FROM search_product WHERE TERM = %s LIMIT 1", $search_term );
   if ( $wpdb->get_var( $sql ) ) {
   

3. Again in this code, you should’ve not used TERM = '".$search_term."'. So I’d replace this:

   $sql = $wpdb->prepare(
   "UPDATE search_product SET COUNTER = COUNTER + 1  WHERE TERM = '".$search_term."'");
   

   with this:

   $sql = $wpdb->prepare( "UPDATE search_product SET COUNTER = COUNTER + 1 WHERE TERM = %s", $search_term );
   

4. And the code after the // if not exist in the database then insert it can be rewritten to:

   else {
       $wpdb->insert( 'search_product', [
           'TERM'    => $search_term,
           'DATE'    => current_time( 'mysql' ),
       //  'COUNTER' => 1, // <- uncomment if the default value of the column is not 1
       ] );
   }