Yes, the generated name of the interface differs if the second connection from the same user gets established.It will be etc., so the way you create the interface and queue names, you’d be creating another queue with the same name, which would obviously fail. And if the additional connection would go down first, you would remove the queue for the surviving connection, also not good.
A way out could be to use the $interface variable (which holds the id of the interface being processed, not its name), to retrieve its actual name (see this very useful post by @Pada), but you have to try whether the $interface value is available also to the on-down script.