Remove duplicated virtual functions from using-declarations

[ZedThree]

struct Base {
  /// Zero arg impl
  virtual int foo() { return 42; };
  /// One arg impl
  virtual int foo(int x) { return x; };
};

struct Derived : public Base {
  using Base::foo;
  /// One arg override
  int foo(int x) override { return x + 1; }
};

This snippet generates duplicate declaration warnings in Sphinx, ultimately due to an issue in Doxygen. This PR cleans up the member list by checking if multiple members reimplement the same refid, and removes duplicates.