How can Harry be a Horcrux? [duplicate]

Question

Since Voldemort has already broken his soul into different Horcruxes before attacking baby Harry, there is no part of a soul left inside him to create a new Horcrux. How can Harry be a Horcrux?

Answer 1

Your question is based on an incorrect understanding of Horcruxes. In Chapter Twenty-Three of Half-Blood Prince, Slughorn explains Horcruxes to Tom riddle (my emphasis):

"Well, you split your soul, you see," said Slughorn, "and hide part of it in an object outside the body. Then, even if one's body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged. But of course, existence in such a form..."

So creating a Horcrux doesn't remove your soul; it removes a part of your soul. The rest of your soul remains in your body. As stated later in the same chapter (my emphasis).

"I am glad to see you appreciate the magnitude of the problem," said Dumbledore calmly. "But firstly, no, Harry, not seven Horcruxes: six. The seventh part of his soul, however maimed, resides inside his regenerated body. That was the part of him that lived a spectral existence for so many years during his exile; without that, he has no self at all. That seventh piece of soul will be the last that anybody wishing to kill Voldemort must attack — the piece that lives in his body."

So each time you create a new Horcrux you split off another portion of your soul, but you still retain part of your soul in your body. When Voldemort tried to kill Harry, another piece of his soul broke off but he still retained the remainder in his spectral form and later in his regenerated body. In Chapter Thirty-Three of Deathly Hallows Dumbledore explicitly states that it was a fragment of Voldemort's soul that broke off and latched onto Harry (my emphasis):

“Tell him that on the night Lord Voldemort tried to kill him, when Lily cast her own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort’s soul was blasted apart from the whole, and latched itself onto the only living soul left in that collapsed building. Part of Lord Voldemort lives inside Harry, and it is that which gives him the power of speech with snakes, and a connection with Lord Voldemort’s mind that he has never understood. And while that fragment of soul, unmissed by Voldemort, remains attached to and protected by Harry, Lord Voldemort cannot die.”