Minecraft Two names one server

The person here is two times on this server online!And that is a Minecraft Premium Server!This bug goes everywhere on this video you can see it, sorry the video is german because this video isn´t from me!But you´ll see two persons!You can skip to 1:08 and you see it!That isn´t a bug of this server!Beacause it works everywhere!
https://www.youtube.com/watch?v=F6hrboUKooc