I've noticed that my client-side UpdateUserTitleDisplayName call still allows duplicate display names despite the "Allow non-unique player display names" option being disabled within the settings. I think this might be a bug. See the image below.
One other strange bit of behavior is that if I try to change the display names via the UI on the backend it will indeed tell me that the display name has already been taken as I expect it to.
Thanks for any help in advance!