Hi Kieran,
I have some notes here please.
1) WooCommerce default behavior is to add the same item quantity to the same item in the cart, so adding 1 after 1 will be 2 in the cart, and that’s why you’re getting the message, you can customize this with some PHP tweek or a plugin.
2) The message is customizable from the Maximum Quantity tab, you can add variables (like product name) there the way you want.
3) Having default & maximum of 1 might cause other issues, because you’re simply forcing two settings for one purpose, you can just set the max to 1 (default always goes 1), or use Allowed quantity of 1 as well.
I hope this helps, please let me know if you need any further help.
Omar