C++ diamond problem – How to call base method only once

Most likely this is a XY problem. But … just don’t call it twice.

#include <iostream>

class Creature
{
public:
    virtual void identify()
    {
        std::cout << "I'm a creature" << std::endl;
    }
};

class Swimmer : public virtual Creature
{
public:
    virtual void identify() override
    {
        Creature::identify();
        tell_ability();
        std::cout << "I'm a swimmer\n";
    }

    virtual void tell_ability()
    {
        std::cout << "I can swim\n";
    }
};

class Flier : public virtual Creature
{
public:
    virtual void identify() override
    {
        Creature::identify();
        tell_ability();
        std::cout << "I'm a flier\n";
    }

    virtual void tell_ability()
    {
        std::cout << "I can fly\n";
    }
};

class Duck : public Flier, public Swimmer
{
public:
    virtual void tell_ability() override
    {
        Flier::tell_ability();
        Swimmer::tell_ability();
    }

    virtual void identify() override
    {
        Creature::identify();
        tell_ability();
        std::cout << "I'm a duck\n";
    }
};

int main()
{
    Creature c;
    c.identify();
    std::cout << "------------------\n";

    Swimmer s;
    s.identify();
    std::cout << "------------------\n";

    Flier f;
    f.identify();
    std::cout << "------------------\n";

    Duck d;
    d.identify();
    std::cout << "------------------\n";
}

Output:

I'm a creature
------------------
I'm a creature
I can swim
I'm a swimmer
------------------
I'm a creature
I can fly
I'm a flier
------------------
I'm a creature
I can fly
I can swim
I'm a duck
------------------