Anti Nickthief
Some leechers use NickThief - this feature takes the name (or parts of the name) of other clients and sends them back to use their community boost, if present at all (eMulePlus is a common example of a community mod). Anti nickthief detects this
The whole Anti-NickThief feature is not about detecting clients with the same name but clients using a nickthief. Though it is annoying, this is not harmful in any way to have cleints that have the same nick. But clients using a nickthief will for 99% also use other bad features like a very low sessionupload, fakeranks or even no ratio/0 UL. That's why one probably wants to detect (and ban) them. the nickthief is very easy to detect. The AntiNickThief sends a random string to each user like "username [abcde]", if the user will adapt that string to his name and sends it back, we can (safely) ban him.
Note that there is also an anti-anti-nickthief Bad feature that tries to detect this naming convention. There is no honor amongst thieves. Though, newer versions of the Anti-NickThief send 2 tags and/or tags of random length to make it as hard as possible to escape the ban.