This mod adds a warning notification message like "You have previously purchased item 'XYZ'. Please remove it from your shopping cart if you do not want to purchase it again". when a customer puts an item into their cart that they've previously purchased.
The customer does have be logged in for it to work (since that's the only way to know who they really are) - the mod checks prior purchases made using the email address of that customer. If a customer is not logged in, then nothing would be checked.
This is for Cs-Cart version 4+ only