The characters refer to a turkish man named Osman Efendi as Mr. Efendi but Efendi already means mister in turkish so they called him Mr. Mister. The correct usage would have been Mr. Osman.
The killing of the police officer elicits no appreciable reactions. Jack and Phryne do nothing more than mention it, Hugh's disposition remains unaltered by it (a fellow whom he must have known in the department), and Dot - especially in that she herself is about to marry a policeman - has no reaction to the danger of possibly losing Hugh in such a violent and sudden way.