How to create a "class set"

While trying to create an Entity-Component-System in C++, I have faced some problems regarding by lack of knowledge on the language.

With a class Entity, that holds the interface IComponent (which acts more like a flag that says "I hold data"), I have a method Add that adds a Component to the Entity if there is not another IComponent of the same class already in it.

Here's an oversimplified sample code:

struct IComponent{};

struct Foo : IComponent{ int x;};
struct Bar : IComponent{ int y; };

class Entity{
    vector<IComponent> entityComponents;

    void Add(IComponent componentToAdd){
        if("entityComponents" does not contain the class of "componentToAdd")
            entityComponents.add (componentToAdd)

    }
}

My expected result would be

Entity e;
Foo f;
Bar b;
Foo anotherF;

e.Add(f); // Works
e.Add(b); // Works
e.Add(anotherF); // Does not work because another 
                 //item in the array already inherits Foo

But I do not know how to get the base class of Foo and Bar from inside the IComponents list and check if they are repeated