Google's John Mueller said that if you have a web site that deploys both a separate mobile URL and also has a responsive design, it can lead to confusing Google. He said on Twitter "We'd probably get confused - which one should we show when? It's unclear."
The webmaster wanted to have both options for "better user experience." But in this case, Google is saying, forget the "better user experience" and keep it simple I guess?
Here are the tweets so you can see the context:
Why would you have alternate mobile pages and a responsive design?
— 🍌 John 🍌 (@JohnMu) March 25, 2019
We'd probably get confused - which one should we show when? It's unclear. I'd strongly recommend using a single-URL setup (responsive or dynamic serving).
— 🍌 John 🍌 (@JohnMu) March 26, 2019
So since it can confuse Google, John is telling the webmaster to pick one. Either go responsive or do dynamic serving but keep it to a single URL and not give users or Google multiple URLs. Forget what is best, or what you think is best, for your users here. But clearly John believes a single URL is best for your users.
Forum discussion at Twitter.