You can't have two different Aethyric Gods with identical domains, because Aethyric Gods are their domains, but you can have a Mortal God and an Aethyric God have similar domains - I believe that's how Kurnous and Taal can both exist. Kurnous is an Aethyric God, and he hangs out in the Heavens of the Cadai, while Taal is a Mortal God that's present somewhere in the Old World, possibly somewhere in the Great Forest or possibly in Athel Loren.
And then you have Sigmar, Mrymidia, the Lady, and the Widow, who can all be described as Gods of "Civilization", but they are gods of specific civilisations, so there's no actual overlap between them.
(Except in the southern Empire, where Mrymidia is starting to gain traction as the Goddess of Civilization In General, which is probably concerning but not our problem)