Should I buy the exact same RAM if I'm upgrading?

Yes, you should absolutely get another 8GB stick of the same model if possible. Mixing sticks is usually fine, but the safest, most reliable path is identical DIMMs. If you can’t find the exact same model, get another 8GB stick with the same specs: speed, CAS latency, and voltage. If you’re already having trouble finding a match, consider buying a 2x8GB kit and selling your single stick.