This won't happen. That's case sensitive (and also accent sensitive).
Your issue is accent sensitivity, but I don't think you want "Hoffi" and "HOffi" to be different people. What you want can really only be done with a custom collation I believe. Alternatively, you may want to suggest accent-sensitive, case-insensitive collations to MySQL.
MySQL's collations are accent insensitive. This means that to them, when you say ...WHERE username = 'Hoffi' it will return a row with a value of "Höffi". Removing the unique constraint on usernames (which would technically allow this) could potentially create security issues or at least create broken situations (places where people are unable to login due to MySQL not returning the correct data).