Joined: Jan 2010
Posts: 9,808
Likes: 1,781
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
I would first check to see if both pulleys turn freely.
I don't really know if the Duopar does or doesn't respond to changes in chain length in the same way that any other particular derailer does.
However, there are two ways that you might be able to test the effect of a longer vs. shorter chain, without actually messing with the chain itself.
You can of course shift the front derailer back and forth.
And (if your bike has horizontal dropouts), you can move the rear axle in both directions.
Hopefully, yours will work as well as the Duopar on my Grand Prix that moves the chain over a 6s 14-34t freewheel (quietest and most responsive friction shifting I've yet experienced).