Ben Bova’s 1976 The Multiple Man is a standalone near-future thriller.
Americo “Meric” Albano is a true believer in President James J. Halliday. Meric is proud to be press secretary to the first president in memory who will be able to do the job properly. After decades of inept and crooked administrations, he believes that the US desperately needs Halliday.
Meric is understandably upset when Halliday is found dead in an alley behind the building where the president gave a speech.
Except… the dead man is not Halliday. It’s simply some stranger who is identical to Halliday … right down to his finger prints.