This is actually a difficult question. My answer is that I would never depend on being able to create two accounts like daveb and daveB. And as a system admininistrator I would prohibit it on all systems that I administer. But as a programmer, I would insure that programs work correctly iif two such accounts are encountered.
Neo mentions mail and that is correct. Internet mail must ignore case and so
daveb@xyz.org and
daveB@xyz.org are the same address. The mail problem would not be impossible to solve but it would be very hard. One of the accounts will need an external mail address that doesn't match the user name. This may require writing a custom delivery agent. I suspect that "mailx Neo" and "mailx neo" would both work. But if it doesn't work, I certainly would not want to call the vendor and complain. And in any case, "mailx neo@localhost" and "mailx Neo@localhost" could not both work.
I'm dating myself here, but terminals originally were not multicase. They were upper case only. Unix had support for that environment. It is mostly gone, but some versions still have various pieces left. On HP-UX, if you sign on as DAVEB, the system will assume that you meant daveb and that you are using a terminal that cannot handle lower case.
Because of these problems, many adduser programs will disallow this sort of thing. But if you manually edit the passwd file, you will probably succeed in getting it to work.
So don't do that, you will regret it. But don't assume that no one else has done it. And be aware that many programmers have assumed that no one has done it.
12-13-2003
