Hi
Better make the following changes to the waitingusers table
In Root/administrator/components/com_virtuemart/install/install.sql line 1340 & 1341
replase
KEY `virtuemart_product_id` (`virtuemart_product_id`),
KEY `notify_email` (`notify_email`)
with
UNIQUE KEY `product_and_email` (`virtuemart_product_id`,`notify_email`)