The Duopar is a great derailleur. I had originally had one on my touring bike when it was still friction shifting. It was mated to an old Suntour 14-34 freewheel with a Shimano IG chain and Shimano 8 speed barcons and the shifting was undoubtedly the best I have ever experienced from a friction bike.
I like how you can simply disengage the parallelogram spring by pulling it away from the tab stop. It's a nifty little feature.