In my current Large Map / Epic game, I currently have 18 heroes named "Viy", one Janusk, and one other Janusk-variant. A bit much perhaps?
This alone may just be a design oversight rather than a bug, so here's where the bug comes in:
All of these units share the same nameplate, even if their unit name is changed. If I change the nameplate on one of them, it changes for all of them. There is no easy way to distinguish these variant heroes. It also changes all of them in other cases than manually changing one nameplate. For example, if I buy a mount for one of my Viy/Janusk heroes, the nameplate for all of them also changes to show a mount, even if that particular unit has no mount.
A similar nameplate issue happens with normal units as well. If I design a new unit type, the nameplate for all of my previous normal units changes to that new nameplate until a save/reload is done, whereupon it goes back to normal. As far as I know however, the Viy/Janusk nameplate bug has no workaround.