I haven't seen this before. I wasn't aware there was a different rate available for single occupancies unless in a room designated as "single". There are so few hotels with single rooms - Doubletree Greenwich in London being the only one I can think of at the moment. However the quote you have says that the second guest stays free anyways. For HH benefits such as complementary breakfast I do know that this is limited to registered guests in the room (not that it would usually be checked if there were 2 people appearing for breakfast from the same room at the same time).