[Not a bug] single product (only one of) can be added to cart multiple times

Website URL: http://www.breadandbutter.co.nz

This website sells products that are generally one of a kind and once purchased are removed from the catalogue. However, when a product is added to the cart and the customer returns to continue shopping they are able to add the product again (multiple times) even though it is a one off item and thus creating a negative stock level (ie; -1 products in stock). As the product is unique it is not possible to supply more than one.

Is there a way to avoid this happening?

This is not a bug.
Check out the settings for Stock in the admin >> Configuration >> Stock menu. One of the options is Allow Checkout. I venture that yours is set to True. Product status should be set to 0 as well. Adjust the others as needed.

Thank you for your prompt reply. Silly me :blush: Fixed.