It's a common phrase in leadership material, but duplicating yourself means to train someone (or multiple people) to be able to do your job as well or better than you. A scary thought, "training your replacement," but it comes down to your motivation for leading.
Refusing to apprentice someone means that if you leave, there is an un-fillable gap. Your absence bring the entire organization down and will take months to get back up to speed. The motivation for this scenario is either insecurity or pride. You fear that if someone is "as good as you," you'll lose your job. Or, you love the idea that only you can do what you do and "they'll miss you when you're gone."
Both selfish reasons.
But to train someone up that can fill the void in your absence is purely selfless. It does not benefit you at all, but is immensely useful for the organization. You do it because it's the right thing to do, and it shows integrity as a leader. Selfish, or selfless. The choice is yours.