Star Trek: Stargazer, book 3 : Three
Michael J. Friedman2003

Identical twins Gerda and Idun Asmund lost their Human parents early in life and were raised as warriors on the Klingon homeworld. They were taught to face every danger shoulder to shoulder - regarding each other as the only certainty in a dangerous and uncertain universe.

The Asmunds continued to depend on each other as helm officer and navigator on the Starship Stargazer, peril and adversity forging a bond between them as strong as tritanium.

But that bond is tested when a transporter mishap deposits a mysterious visitor on the Stargazer -- a beautiful woman from another universe who resembles Gerda and Idun as closely as they resemble each other.

As Captain Jean-Luc Picard pits the Stargazer against a savage alien species in a gallant attempt to send their visitor home, Gerda comes to suspect the woman of treachery. But she has to wonder -- is she following her Klingon instincts or succumbing to simple jealousy?

Gerda needs to find out -- before Picard and his crew pay for their generosity with their lives.