Shemeska
Adventurer
Unless of course they are not 'concepts' but rather supernatural intelligences drawing power from those concepts. For instance a God of Fire doesn't necessarily control Primordials of Fire (such as Imix or Surtur) and vice versa, so there is no reason to assume that a God of Fire is 'fire' itself.
Again, that boils down to how precisely you define what a god or similar being actually is on a basic level in your own campaign setting.
Likewise your idea doesn't allow for the possibility of two Gods of Fire coexisting simultaneously.
It hasn't stopped me before. Imix and Kossuth just to pick a pair, both gain worship as 'gods of fire', and both openly claim to be just that. Assuming for the moment that they both are indeed gods of fire, and physically embody that concept -indeed -are- that concept itself- you might be right, except despite what they might claim, or their worshippers' believe, they're each embodying different aspects of the same concept.
Secondly, even if they embody the exact same concept like two deities created from the gestalt worship and belief of mortals on two different worlds on the material plane, I still see little reason to not allow one of them to exist, or have them be the same thing like you would imply. They might be the same concept, but the same concept as envisioned and viewed through the lens of devotion of two perhaps very different cultures.
Another example: demons being a physical manifestation of malignant chaos hasn't stopped the Abyss from being populated with an uncountable or infinite number of them. There's no limitation imposed at all like what you mentioned. It's a very different style than your own, but I've never come across anything restrictive or limiting (rather quite the opposite).