Yes, but you could just have the *ONE* route through DUS, and do all of the rest of that on your own, picking up the *ONE* again in DUS. In many areas, it should be pretty easy to build a nested itinerary where you use the *ONE* to get the major distances, and then book independent segments/round-trips on local/regional airlines. For example, you can get ARN-BUD on either Sterling or Malev as non-stops or FinnAir or Blue1 through HEL. Then you don't use any segments against the RTW ticket and still get to make the routing you want.

If I was looking at one now, I'd be looking at the *ONE* ticket as the way to get from continent to continent and then local airlines for shorter distance tickets. You could end up better from the travel perspective when you start thinking about those 20 segments as your available balance for long-haul flights.