Basically compared to Skype, Facetime, Whatsapp, Facebook, Signal, Telegram, etc. Duo is an also-ran that most users of the before mentioned competitors have probably never even heard off. I understand they managed to get some traction in some secondary markets that are mostly not the EU or North America. I actually have conversations with people on all of these, except Duo. I've never had a single person reach out to me with the request to do anything whatsoever on Duo. Not a thing. I've never seen it in the wild. I don't know a single person using it.
I just came out of a meets/hangouts meeting. I'm sure the rebranding is very significant to Google internally while they deal with Conway's law and internal teams continuing to try to "own" this space. But from the outside it looks like more of the confused strategy around Google Voice/Hangouts/Docs/Buzz/Wave/etc. where there's always an orphaned thingy that is actually useful that vaguely enables people to actually talk and chat to each other.
Meanwhile, between Slack, Microsoft Teams, Zoom, Google seems to be lacking a good strategy for competing effectively with those. I find it very telling that startups that are Google customers still opt to use things like Slack and Zoom. There's something missing there; a vacuum that Google has been dancing around for years without actually managing to compete. Literally every team I've been on in the last five years had Google docs + gmail as well as Slack.