Not sure if it's already been mentioned, barca have been wearing and advertising double denim for a few years, they use them as travel gear. Looks very naff in my opinion.
Nothing wrong with double denim so long as the colours or shades are clearly different, other wise it looks like a denim suit, at least that is what the sales assistant in the Levi store in Churchill Sq told me when I bought a blue denim shirt and black denim jeans (I am also a big fan of Status Quo.......)