always there theory
The "Always There Theory" (ATT) is a time travel theory. In which the time traveler already exists in the past. If you turned 20 years old today and went back in time to a week ago, the first instance of your 20 year old self would actually exist before your birthday.
One reason I like this theory is because it's actually time travel (and not just cheating by making a new timeline), but it also would prove fate.
If your future self walked up to you right now, and they were ten years older than you, then you would be invincible for 10 years. You could do whatever you want and could never die. Because if you were to die during those 10 years, you wouldn't be able to time travel and thus never meet your future self.
A downside of this would be that you have no free will. As your future self has done everything you're about to do. Your future self could tell you everything you've done in those ten years, (including being told what they would do in their ten years) and you would do everything they've said. This is a con, that you cannot change the past, as the changes have already been made.