OK, I re-read your post and noticed that your itinerary contains both BA flight numbers and AA flight numbers. I assume that one of the flights is an AA codeshare on BA metal. BA created one PNR for pure BA flights (carrying BA flight # in your e-ticket) and a separate (operational) PNR for the BA flight booked as AA codeshare (I assume it must be the LHR-LAS flight). If your DUS-LHR flight is booked as a pure BA flight number nothing should stop you from checking in online - just try it. You should also be able to check-in online for the LHR-LAS flight if you use the 'operational; PNR number.