OP means original poster on a forum. The eccentric is that round chunk of aluminum with the offset round hole in it. My Burly Bongo uses the same size eccentric as your Burley Duet. Eccentrics are not only used on tandems but also on singles. My Co-Motion Pangea Rohloff uses an eccentric with a threaded offset hole in it. This makes it simpler to find replacement parts. It also uses an interior locking system. I believe you should replace both the spindle and the bearings on your rear BB. That spindle should not move. My front BB spindle bent because it was under engineered for my uses. I weighed 190lbs the wife was close to that having just had the third child. I was also pulling a custom made bicycle cart that was designed to haul up to four children and groceries. Yeah All three were in the cart going up a canyon. Not only did the cheap spindle bend but the rear bolt on hub axle bent on those cheap hubs. Broken spokes also. Burley sent me hub axles and spokes twice. I was working my first good job and ordered in two pair of Phil Wood 48 spoked tandem hubs. I built up those wheels and the only problems after that were cracking rims on occasion. The exterior eccentric retention system, you no the two grub screws. Didn't always hold and I had to keep tools on hand to readjust.