// only add one force sell product to the cart no matter how many of the original product are added
function my_wc_force_sell_add_to_cart_product( $product ){
	$product['quantity'] = 1;
	return $product;
}
add_filter( 'wc_force_sell_add_to_cart_product', 'my_wc_force_sell_add_to_cart_product' );

// when a synced force sell product is updated always set it to 1
function my_wc_force_sell_update_quantity( $quantity, $product ){
	return 1;
}
add_filter( 'wc_force_sell_update_quantity', 'my_wc_force_sell_update_quantity' );