How do you get from boss duel to final in sonic and the secret rings?

You're probably thinking of the "Final Boss" fight in Sonic and the Secret Rings, which is not actually a separate level from the boss duel.

Here's how it works:

1. The boss duel: You fight the "Final Boss" in the "The Last Story" level.

2. The transformation: After defeating the boss, Sonic transforms into his "Super" form.

3. The final battle: The "final boss" reappears in a new form, and you fight it again in the same level.

So, you don't progress to a separate level after the initial boss fight. You simply continue the battle against the "Final Boss" in its new, more powerful form after Sonic transforms.