explicit specialization; 'std::hash<_Kty>' has already been instantiated

first up, the code

"file.h"

namespace doge
{
    struct A
    {
        void func() = 0;
    };

    struct Aa : public A
    {
        std::string id;
        void func() override {}
    };

    template<T>
    struct Aaa: public Aa
    {
        T data;
        void func() override {}
    };

    class D
    {
    private:
        std::unordered_map<Aa,int> intMap;
        std::unordered_map<Aa,std::string> stringMap;
    };
}

namespace std
{
    template<> struct hash<doge::Aa>
    {
        std::size_t operator()(const doge::Aa & key)
        {
            return hash<string>()(key.id);
        }
    };
}

I'm getting the error in the title which seems to suggest multiple code for the hash has been generated. I get 2 of these in my error message, one for each unordered_map, can someone help me identify the problem. Thanks.

explicit specialization; 'std::hash<_Kty>' has already been instantiated