This in an interesting discussion, especially if you also consider that there was another discussion about doppelgangers a few days ago. I would suggest to combine a bit of both ideas and do as follow :
- for some troops whose name or characteristics implies uniqueness enforce such a thing. I mean, THE great maw, the name implies there is only one, but same could be said about all named characters like Emperor Khorvash or Cthyryzyx. In addition to all the generic units like Peasants, Serpents, Revenants and so on, some troops like Behemoth, Bone dragon, Crimson Bat, Hydra, Shadow dragon and a few others could be allowed as multiples despite their rarity because their names does not imply uniquess. So, uniqueness would have to be applied thoughtfully, on a case by case basis, not just considering the name but also taking into account game balance (note, there is already a unique troop in the game, the hero we are playing)
- then with the introduction of this uniqueness, a Doppelganger unit would be quite interesting, precisely because you could then duplicate some of your unique troops to achieve some kind of desirable effects that could not be achieved straight away.
Those two combined game mechanics would also open a new space of trops design, allowing for example to create new troops that would be powerfull when duplicated in a team but with uniqueness restriction so that a Doppelganger would be required to achieve the desired result.
But all that is hardly a necessity, and I guess the devs already evaluated those options since those ideas are far from new or really original ones. Yet, since there was a discussion here, that was an opportunity to speak about that.