We all know that Google can handle crawling and indexing all sorts of sites, including many types of mobile sites. But if you are using both an m-dot, i.e. a separate URL, and you are using a responsive design at the same time, that can be super confusing for Google. It is basically telling Google you have two versions of your mobile site and how can Google pick which one to use?
John Mueller from Google said on Twitter that if this is your set up, then "I'd try to clean that up." He said you can have some URLs that are only m-dot and some URLs that are only responsive, but if the same page is both m-dot and responsive, it will be hard for Google to know which URL to rank and how to consolidate the signals.
John said "if you have both responsive & m-dot for a page, it might be hard for our algorithms to figure out which one to show to mobile users."
If you have both responsive & m-dot for a page, it might be hard for our algorithms to figure out which one to show to mobile users (even outside of mobile-first indexing)... So yes, I'd try to clean that up :)
— John ☆.o(≧▽≦)o.☆ (@JohnMu) May 9, 2018
Here is more context:
It's per-page (sites can be mixed), and it depends on what we see as the mobile version of that page.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) May 9, 2018
Forum discussion at Twitter.
