Duplicates

Duplicates are replicas of the victims of Trin's unique magic. She creates the duplicates by converting someone's soul into a shell that she commands. The race of the target isn't a hindering factor, but the target can't be too well-shielded and has to be in view.

The number of copies that she can keep at the same time depends slightly on her relationship level with Simon: At certain points in the story she will be able to free up slots by releasing copies once they're no longer needed.
 * She starts out with 6-8 copy capacity at the start of Aka's route: 6 guaranteed, plus additional 1 at 30 and 50 affection.
 * Upon reaching the airship negotiations in Chapter 3, she will gain 1-4 copy capacity: 1 guaranteed, plus additional 1 at 50, 75, and 100 affection.

Even if the party doesn't kill or imprison a previously copied person as a potential loose end, the targets generally suffer degraded physical and mental condition from being copied for a prolonged period.

Jade clerks
During Aka's route, Janine asks Aka to sabotage House Jade, so the party uses Trin to make the Jade clerks into sleeper cells. The more Jade clerk duplicates there are, the fewer the mercenaries (enemy forces) that House Jade can send against the Doom King's army during the invasion of Yhilin. With the exception of the Aramite Jade Clerk, all the clerks are dismissed after House Jade is finished off.

The relative worth of each clerk for the goal of sabotaging House Jade is listed here.

Required duplicates
Currently the only one is Yuanuon, who is copied at the end of the Zirantia trip in Chapter 3, and is not removed so far (beginning of chapter 5). If you do not technically have a free slot for him, he will be added anyway, but this technicality does not seem to be exploitable.

Temporary duplicates
These are duplicates created during short scenes, who don't count against your total in any meaningful gameplay way.
 * During the coup attempt, Trin and her conspirators copied members of the palace guard.
 * Reval was duplicated and eliminated during Megail's quest in Ari-Yhilina.
 * Trin temporarily duplicated Altina and herself during sex scenes.
 * Members of the Divine Claw will be duplicated during the Ordeal, while Wynn wipes their memory.
 * Unless FANCY WALK was funded, Trin will copy some coup solders while defending Yhilin Palace during the Third Arclent War.
 * The Lecherous Priestess will be copied just long enough to get information from her, regardless of whether you choose to copy her long-term.