(Noticed in 0.75, I've not downloaded 0.80 yet.)
Looks like heroes can be selected more than once by the same faction (and I've strong suspicions they can appear more than once for different factions as well).
The context: I had a small outpost located between sea an a montain range. It was a natural invasion road for the opposing faction (magnar) so I garrisoned some troops and the troll champion here. Magnar keeps sending wave after wave of lone champions, quite easily dispatched. I noticed that this 'Mergil' came sometimes on a mount, sometimes without, and the duration between two assaults was inferior to the required 5 turns.
I finally understood what was going on when I took the city Magnar had settled near this contested area: there was TWO of them ! 
Not really annoying nor game breaking, but, eh, kinda strange anyway ! 
I think I spotted this for another hero: I got the Ulfnir champ and, while exploring the continent, i'm kinda sure I crossed the path of an Altar stack whose lead was aother Ulfnir. I cannot confirm this, Altar resigned shortly after.
