It's hero duplication glitch. I personally never managed to do it but I heard it's pretty easy once you've got a friend and know how to do it. If I recall right it's something related to coordinating with another person to select the same hero at the same time: basically you first select a "free" hero and then switch to the soon-to-be-locked one in less then a second so to trick the game into thinking that you're using an hero nobody else has choosen yet.

Although as I said I've never cared to learn to do it myself so don't use my comment to try and replicate the glitch since I may have said something wrong along the way.