Somehow able to start game as any name in offline mode

My friend recently discovered he could run several instances of Minecraft on the same computer, and log them all into one LAN server. He starts up a world with a standard account, uses the "Publish" command. Next, he starts up a second instance with any username and no password, triggering the "Play Offline" button. He then joins the LAN server with the offline account, and that's that.

What differs is that on his computer, the username he picks for each offline instance is kept when logging in, while on my computer, I log in as "Player" no matter what username I pick. This results, as seen in the screenshot, in instant LAN parties with no extra accounts on his end, while I can only play with a single extra person, as logging in as a second player kicks the first of the same username off the server.

This friend has not modified his client in any way, and neither have I. We both have tried it in the same versions, with fresh installs of the game.

I do not know if this is a security issue, as you obviously can't join multiplayer servers as a non-authenticated account, but just in case it is, I have set the issue to private.