HEIR APPARENT by Tyree Campbell

Answering a distress call, March and Myrrha find a young woman who has
deliberately been marooned on an uninhabited world. She claims to be Hoya
Palologa, heir to the Palologa throne on Wanderby. But there is already a Hoya who has been invested as the heir apparent to that throne. Myrrha believes the claim of the Hoya she and March have encountered. Thus begins a journey to establish the succession, a journey made far more perilous because Hoya not only claims the throne, but is also a sinister personage with several crimes on her resume.

March and Myrrha find themselves embroiled in internal politics on Wanderby,
where the slightest wrong move can get them killed. The rulers on that world are oblivious to the subtle machinations of their underlings, one of whom has created a lookalike but false Hoya. Which one is which? And will death take the real one before March and Myrrha can stop it?