If there's no play on the NDS crank after tightening the bolt, then try using blue loctite on the bolt threads. If there is play, then the crank and BB should be replaced. You can only replace the NDS crank. But sometimes its difficult to get the same make/model as a single crank. I ended up replacing the crankset and BB the last time I had a similar problem. You could also replace with a cheaper used crankset to save some money, worse case. In my case, I found a good deal on a Campy Centaur crankset and just went for that (used to have Chorus).