Display name already exists is a long standing bug/glitch.
The fix is and always has been to just Export > Delete > Import
We don't know why it happens or what might cause it, it almost seems random. Since we cannot cause a machine to do it, we can't fix it, I'm sure the engineers have tried, but the problem is, it's so easy to resolve (less than 2 minutes) that for most of the customers who have had the issue, when they call in, they don't want to take 30 or more minutes to collect data logs and config sheets and all the other stuff that would be required by Engineering to diagnose/fix it. And there is no certainty that they could, because possibly the logs would show nothing wrong. And it could take days/weeks/months to figure out, and you couldn't verify the fix unless you left it broken for that days/weeks/months time span.