Update: the bug is fixed.
Original article: Dota 2 patch 7.32e introduced several changes in the game and created new bugs. One of the biggest issues is doubled heroes during the swap. This way, you can have two identical characters in the same game.
The copy appears when you swap a character with another player. According to Dota 2 bug tracker and Reddit posts, If a player accepts a swap, his hero duplicates the original which remains the same.
Apparently, it only works in Turbo, as all the bug reports mention it. The screenshots of the games also all show this mode.
Turbo mode received multiple changes in patch 7.32e. The update came out on March 6-7, featuring item and hero adjustments, the new hero Muerta, and a mini-game related to Dead Reckoning chests. Valve promised to release a bigger update in late April.