I thought I'd add my bike to the jubilee clip club:
As you can see, only one clip. Time will tell how well it will hold up, but so far, rock solid. I've tested it with some heavy groceries over pot-holed streets, and all seems well.
If this works well, I will likely do the same thing to my upcoming cargo bike as well... Very handy if you already have invested in Brompton luggage.
PS. Sorry about the funky rotation, uploader doesn't like vertical iphone pics, apparently.