QUOTE(SPoison @ Aug 16 2019, 23:27)

50%
Thanks.
If that's the case the description for the perks are wrong, they imply that the bonus given stack giving an increase of 270% damage if all unlocked.
The description for the perks should be changed to this:
Dæmon Duality I Increases attack damage and magic damage by 10%.
Dæmon Duality II Increases attack damage and magic damage from 10% to 15%.
Dæmon Duality III Increases attack damage and magic damage from 15% to 20%.
Dæmon Duality IV Increases attack damage and magic damage from 20% to 25%.
Dæmon Duality V Increases attack damage and magic damage from 25% to 30%.
Dæmon Duality VI Increases attack damage and magic damage from 30% to 35%.
Dæmon Duality VII Increases attack damage and magic damage from 35% to 40%.
Dæmon Duality VIII Increases attack damage and magic damage from 40% to 45%.
Dæmon Duality IX Increases attack damage and magic damage from 45% to 50%.