Markle, Merkel -- it's almost necessary to use each lady's first name, lest one of them finds herself on the front pages with a most unsavory headline: "Merkel's Dad Meets Prince Harry!" or "Markle Addresses Donald Trump's Victory With German Citizens."
See? A simple mix-up like this could put either party in a sticky wicket.
Therefore, it's imperative to differentiate between the two.