I generally prefer to lurk, but I wanted to review, once again, all the possibilities we have for the permanence of duplicates:
1) Duplicates are permanently permanent, forever. I hope I dont have to explain why this would be OP, but just in case I do, keep in mind that Slately- and his duplicate- are wearing tons of magical artifacts that took Ace and Cubbins many turns to make. If things like this can be duplicated, instantly, permanently, by a single Mancer, then Dittomancers may be the most powerful class.
2) Duplicates are permanent, but are diminished in some way. It's hard to tell if this is the case, but it's possible that this is balanced simply by duplicates being more squishy, or intellectually diminished. Since this is Slately... we'll never know.
3) Duplicates are permanent, and serve as an "extra man", as in video games. In other words, you summon a duplicate, and IF the original dies, the duplicate becomes the new "real" unit and things continue as normal. Otherwise, the duplicate just... hangs around, being passive. Kind of like how dream selves work in Homestuck.
4) Duplicates are temporary, and otherwise work as above. If the original doesnt die, the duplicate ceases to exist when the spells ends. This still would mean that this duplicate is now permanent.
5) Duplicates are temporary, and decay in a manner similar to undead. Slately may cease to exist in a few turns. For all practical purposes, this would be pretty irrelevant. Whatever happens, it seems unlikely that a Slately, any Slately, is gonna survive this turn.
6) Duplicates are temporary, and cease to exist immediately at the end of turn. This is what most forum people seem to be assuming. It makes sense and is in keeping with Lloyd's bleak expression in panel one.
7) Duplicates are temporary, and are directly sustained by Juice. Slately is alive now, but it requires that Lloyd maintain a mental connection with him. If Lloyd is croaked, or runs out of juice, or maybe even leaves the hex... the King is gone, and by extention the rest of the side. This is the worst-case scenario for Jetstone. Assuming the archons know how Dittomancy works (but why should they? no one else seems to understand how various obscure disciplines work), Lloyd just became as much of a juicy target as Slately himself.