It's not strictly a duplicate; it's another overload. Whether or not it's intended for us to have both a single-argument and a 2-3-argument overload isn't a bindings issue -- it's a question for the WebRTC folks.

They are not the same method.
The first one is the modern promise-based version and the second one is a legacy callback-based version.
The callback-based version returns a Promise<void> because it is not allowed to have overloads that return different types. The callback-based version always returns a resolved promise that should be ignored.
See https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-setlocaldescription
and https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-setlocaldescription!overload-1 for the difference.