For the troll sessions, the one we see is the one in which the ectobiology occurred, and duplicates appear to have been split off into the previous session (
reference). The first session failed because both the trolls and their ancestors were actually created through ectobiology in the next session. This glitch (not operating in the session in which you were created) is also apparently a hallmark of Lord English.
This has odd implications for the human session, however, given that the
first session we see (the Sburb Beta session) has the ectobiology that created the kids and their ancestors. No paradox doom, no calling card for Lord English . . . but the same is not true for the Sburb Alpha session.
I'm assuming that the first troll session was doomed because they needed to fail in order for them to initiate the Scratch that created the universe that contained their ectobiology session (we're working with stable time loops here), but the first human session contained
both the Scratch and the ectobiology, so the second session is paradox-free but vulnerable to Lord English's influence (since its heroes were not created in that session). Instead of paradox doom, the Sburb Beta session fails due to the chain of events leading up to Jack's ascension as Bec Noir.
If the human-session cross-Scratch ectobiology worked the same way as the troll-session cross-Scratch ectobiology, then the heroes in the Alpha session would be differently-aged duplicates of the heroes in the Beta session. If this is so, then Jane, Jake, Roxy, and Dirk would be unrelated to each other (being clones of themselves), and their currently dead ancestors would have been duplicates of John, Jade, Rose, and Dave.