As LDAP is case insensitive for usernames, every time one of the users enters his/her username (ex: iurii.furmansky, iuliia.skrypchenko) with a different letter case (ex: Iurii.Furmansky, IURII.FURMANSKY, Iuliia.Skrypchenko, IULIIA.SKRYPCHENKO), a new user is created and the user logins with the default profile permission (Project Guest in our case). This is generating a lot of clutter in the list of users and is causing unneeded support calls from the end users to the admins. In addition sometimes the issue is found late and there is already two users representing the same person maybe with different access permissions. :S
It is common in the several vowels names (Russian, Ukrainian) like the above examples or multi-part names like the Arabic names AboSalah (Abosalah), AbdulAziz (Abdulaziz), ElMansy (elmansy) or names like McLennan (mclennan), Pole-Carew (pole-carew).

So the username has to be stored in the either "all caps" or "all small letters" to be unique.