Is WooCommerce $cart_item_key globally unique ?

I want to make some changes to customer's cart by refereeing and keep tracking of their cart_item_key (Generated by WooCommerce Cart).

Using answer this page : woocommerce how to get $cart_item_key for a single product

Does anybody know if the Key is unique per shopping cart or globally ? Is there a chance of having two item in the cart (from different customers) with the same Key ?

Thanks