
Originally Posted by
DrByte
You could consider writing custom code to trigger (via a notifier/observer) on the Shopping Cart page, which could throw up some warnings via the messageStack about things they already "own". It would need to fire on all checkout pages as well as the login-success stage and redirect them to the shopping cart for displaying those messages, so they can remove the items from their cart ... or click a link in the error message to take them to their My Account screen to look up their order history.
Writing observers is custom code. There are many examples I've posted around the forum, as well as many addons which use them so they don't need to touch any core files when providing their added functionality.