I'm wondering if it's possible to only allow the customer to add an item into the basket once so trying to add it again would result in something like
"this item is already in your basket"
Any help would be appreicated as I've searched high and low for an answer but the only thing I have found is limiting the number of products actually in the cart to 1 which is no good.
The ideal solutions would leverage Magento configuration settings to limit the total item maximum per SKU to one per order though you might need to code a solution if you want to have a custom message presented to the user that is different from the error message displayed by default.
Adding a unique message to be shown to the user would require custom coding or purchasing / installing an existing extension, though I'd start with the solution described here in overriding the observer on the add to cart action.
