On Misty's site, where there are a lot of users, if you want a name that's already taken, you can get around it by doing things like Amαury (notice that the second A is acrylic) or A m a u r y. However, the former -- using acrylic letters or letters with accents over them -- does not always work.
I had requested a name change to Simbα because there was already a user named simba, and Simba would have made no difference, which didn't work. I then asked Misty to try Símba and Simbá, both of which didn't work, so I just went with S i m b a.
Doing personal testing on KH-Flare, I renamed Test Account to Simba. Now, if I also want to be named Simba, and I input it as Simbα, Símba, or Simbá, I get the following error, which says the name's already in use even though it's not: