Mage246 说:
Well, that's odd. Since there's 2 borders actually. One between the lower 48 and Canada, and one between Alaska and Canada.
Yeah, but who counts Alaska as anything but a vacation home for Polar Bears?

The US & Canada have two borders, but one of those borders happens to be the longest border with the US in the world, poor bastards. The point I was trying to get across being that since you can have multiple borders, the use of "single" to denote that this is an individual border is correct. Like saying "the single largest pie in the world" does not denote that you have the only pie in the world, but that of all the pies in the world the pie you have happens to be the largest individual pie.