Preventing same Event handler assignment multiple times

Baget is right about using an explicitly implemented event (although there’s a mixture there of explicit interface implementation and the full event syntax). You can probably get away with this:

private EventHandler foo;

public event EventHandler Foo
{
    add
    {
        // First try to remove the handler, then re-add it
        foo -= value;
        foo += value;
    }
    remove
    {
        foo -= value;
    }
}

That may have some odd edge cases if you ever add or remove multicast delegates, but that’s unlikely. It also needs careful documentation as it’s not the way that events normally work.