HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

This splitting bug also exists in SoD, but it's only visual. Although, it might be a flaw of the mod.

It's good that you brought this up!
I had the same thing happen to me in a clean SoD and offline, a couple of months ago: I was splitting a stack of about 70 Phoenixes, and suddenly, out of nowhere, their number doubled: 70 there and 70 there! "Not a bad bonus to my army!" :) But I didn't cheat; I still won! (I was afraid that it might later lead to the game "crashing" due to a "phantom" stack).
So, it seems that this is still a "native" SoD bug (I have version 3.0 from "Buki").
Does anyone know when it occurs?
Baratorch, it would be good to fix this bug in the HD mod as well, so that the number of "cheating" players doesn't increase!
By the way, if anyone knows, please tell me! I also encountered another unpleasant bug: in battle, I clone a stack standing, say, in the corner, with other stacks around it; the clone appears, but the original stack becomes invisible, although it is still present, and during its turn, the game "crashes"! :(