What mechanism can be used to verify that a participant in an online election is a real person who has only cast their vote once?

For a “company owned by humanity” in which every person in the world has one vote, a method is needed to verify that the online identity belongs to a real person. It is also necessary to ensure that people do not register multiple times. What solutions could be used to ensure this? Which of these solutions could be implemented on a small scale with little effort, and which could theoretically be scaled to the entire world population?