Make display name non-unique
I've been wanting to change my display name for a while, but every time I tried it gave me a not very specific error for it. Turns out, you can't have the same display name as other people. Why? This is counterproducent for those people who just set their given or family name as display names, so those who wish to put their name as "Tanaka" have to somehow change the spelling of their name to be allowed to have it? That doesn't make sense, the display name doesn't even overrid the normal username, so why do they both have to be unique?
Make it either one or the other, or just let display names be whatever.