Let’s say we have a certain Trill symbiont with a host. What would happen if the symbiont was duplicated under the condition that:

• The host and symbiont were transporter cloned. (2 Jadzia Daxs)
• A person from an alternate timeline with the same symbiont ends up permanently marooned in the prime timeline. (Larry Dax from a timeline where Curzon didn’t reinstate Jadzia coexisting with prime Jadzia)
• A past host comes back from the dead with a version of the symbiont a la Spock or Shaxs, or even something similar to Doctor Who’s concept of an extraction chamber (Jadzia got bored in Sto’Vo’Kor and decided to climb the Black Mountain, meaning her and Ezri exist simultaneously)

I imagine in all of them, the commission would at least let the duplicate live for the rest of the lifespan of the original host, much like the Federation at large treats transporter clones.

However, what happens when it comes time for the symbiont to be transferred? I can’t imagine the commission’s ideology would smile upon duplicate experiences under much of the same rationale against re-association: there would be a duplication of experiences rather than the acquiring of new ones.

I think in the first case at least, it is reasonable to assume that they’d begrudgingly transfer both symbionts, as both have the equally valid claim to being the original and randomly killing one is straight-up murder, which I imagine the rest of the Federation would dislike.

They might also do so in the second case, as at least our Larry boy has some different experiences even if some are duplicate with prime Dax.

The third one is where it gets very muddy. The nature of souls in general is a muddy subject - twofold when there are two beings involved. For the sake of argument, we’ll say the Jadzia in Ezri’s symbiont accessible by Zhian’tara is a “backup” of Jadzia up to her death and that a separate Jadzia Dax went to Sto’Vo’Kor¹. What then?

1: I make this assumption because a) Ezri doesn’t have Jadzia’s memories of Sto’Vo’Kor and b) it was the combination of Dax and Jadzia that engaged in Klingon ritual and “just” Jadzia would not be the person that participated. Of course, this starts getting into the more mystical parts of the franchise, and it’s probably good they keep it vague even through it makes canon discussion like this a nightmare… a FUN nightmare.