I'm also interested in what the difference is.
The 'Basic' (first) tab a 'Screen name' field which is the IRC nick. The 'Advanced' (second) tab has 'Username' and 'Realname' fields, the username goes in the IRC hostmask (and usually defaults to the username you have on the machine you are using) and the realname shows up when people request your /whois information.
(The above is all to the best of my knowledge, I don't ever mess with 'Username' or 'Real name' so it is possible I have those wrong.)
If you see both 'Username' and 'Real name' as the same string then your translation is at fault and that should get fixed.