There's no need to complicate things.
Offer Duncan a cut and let him resolve the dispute.

No, for the kind of money that Weidmann demands, the only way to resolve this is for the three of us to meet face to face to face and you bring photo ID, birth certificate, letters addressed to you, etc etc, proving beyond all doubt you are Otto Beck and Martin Weidmann does likewise and then after this evidence is examined by myself and my solicitor, will I cough up the money. You can't lose!
Anyway don't worry Weidmann is rich and will pay for my flight and obviously I will need my solicitor to fly with me, and unless you live in the same city in America or Europe as Weidmann, what are the chances??, he will presumably pay for your flight too!
I look forward to meeting you both and maybe as soon as next week.
JohnM