Was Trump born a dual citizen?
Actually that's a good question. I would say yes. But, prior to 2009, this might not have been as clear as it is today.
Any question the British nationality of children born to British mothers before 1983 was settled by the Borders, Citizenship and Immigration Act of 2009 which provides clarification that any person born outside the UK to a British mother may register as a British citizen by descent if that person was born before 1 January 1983. Trump was born in 1944 which is before 1983.
He can file the form to pick up his UK passport at any time.