Help!

I've just had this error message appear:

Fatal error: Cannot redeclare class ot_quantity_discount in /home2/myusername/public_html/includes/modules/order_total/ot_quantity_discount.php on line 12

It has meant the right hand side of my site has disappeared (where the shopping cart is!).

Does anyone know what I can do to fix it? I've just uploaded new versions of the files again and the problem is still there.

Any help appreciated.

Gary