Advanced query help

I am using a custom query string that uses meta_key and meta_value to sort the results:

$querystr = "
    SELECT wposts.*
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id
    AND wpostmeta.meta_key = 'Model'
    AND wpostmeta.meta_value = '$vehicle'
    AND wpostmeta.meta_key = 'Discounted Price'
    AND wpostmeta.meta_value < '$price'
    ORDER BY wposts.post_date DESC
 ";
$pageposts = $wpdb->get_results($querystr, OBJECT);

This isn’t working at all, am I missing something really obvious. Any help will be gratefully received.