This looks like it dups a paying customer's IP and MAC addresses. Does that work if both devices are running at the same time? I was under the impression TCP didn't like that.
The HN Title is (I can't see the actual article as the machine is hosed):
"How to get Gogo in-flight wireless internet for free"
gogo-in-flight-wireless-internet-free
Or am I missing something?
Yes, you're missing something. The article is a disclosure of a security vulnerability that has already been reported to the company responsible, including notice that it would be published and a request for confirmation that it has been fixed. (Edit: but your response is reasonable - I can see how the article title is misleading.)
Layer2 attacks like this aren't clever, and can be very difficult to prevent because of the nature of communication at that layer.
In a corporate/campus/hardline environment, there are plenty of reasonable mitigation strategies. (802.1x &c).
For a shared hotspot, this type of behavior is just anti-social.