From the table on this page:
- Necromancer Edition has "17 Additional Skins"
- Supporter Bundle has "Additional Skins For All Minions"

Can any owner of Necromancer Edition or Supporter Bundle share the details what is inside these skin packs?

Is "17 Additional Skins" and "Additional Skins For All Minions" the same thing but with a different name?

Simpler question: we own the base game, what must we buy to get all the complete existing content, of course without rebuying what we already own?

Because it seems that we must buy the game 2 more times (Necromancer Edition + Supporter Bundle) for that, which is unfair.

Ditto. I don't own either, but according to this page: https://www.gog.com/en/game/iratus_lord_of_the_dead they are different skins.

Gog added redundant info to the editions table, so it's indeed confusing.

- The "17 Additional Skins" should be the Necromancer DLC ones.

- The "Additional Skins For All Minions" instead.. I'm not sure.
If it's a mistake, they could just be the Supporter Pack ones. Otherwise what, extra skins or the Halloween+Holiday+Summer ones?

From what I understood:

- Necromancer Edition = Base game + Supporter Pack DLC + Necromancer DLC
- Supporter Bundle = Base game + Supporter Pack DLC + ( extra skins? or not?)
- Necromancer DLC = various new content, skins
- Supporter Pack DLC = 1 new minion, ost, art, skins
- Halloween+Holiday+Summer 3 DLCs = skins

To get all content you would just need Necromancer Edition + the 3 HHS DLCs...
..UNLESS the Supporter Bundle really has extra skins.

I contacted support and they confirmed it was a mistake and then they fixed the page.

So it's:
- Supporter Bundle = Base game + Supporter Pack DLC
- full content = Necromancer Edition + 3 HHS DLCs