How do you handle over-dependence on a single key team member

Question

I’m managing a project where one team member holds a lot of critical knowledge and responsibilities, and it’s becoming a risk. If they’re unavailable or overloaded, it could slow everything down. I want to make sure the team isn’t too dependent on one person. How do you handle situations like this—distributing knowledge, balancing workloads, and making sure the project runs smoothly even if key people are unavailable?