Index: order_coupons.sql
===================================================================
--- order_coupons.sql	(revision 576)
+++ order_coupons.sql	(revision 577)
@@ -14,5 +14,6 @@
 );
 
 CREATE INDEX webshop_order_coupons_user ON webshop_order_coupons USING btree (uid, session);
+CREATE UNIQUE INDEX webshop_order_coupons_in_order ON webshop_order_coupons USING btree (source_id, dest_id) where source_id > 0;
 create index webshop_order_coupons_source on webshop_order_coupons (source_id);
 create index webshop_order_coupons_dest on webshop_order_coupons (dest_id);
