You are in an empty room, standing near a wall facing the opposite wall. You roll a ball and measure the time it takes the ball to hit the opposite and reach back to you.
You repeat this again - but now in the middle of the way there is a time traveling portal, and in the other side is the same middle of the room but in 1969. The ball maintained its speed vector in the transition.
How long does it take the ball to reach you?
That’s not the time travel I like because it doesn’t create any fun paradoxes!
I support the “unique universe” idea, with all the paradoxes that that can generate. So, you send the ball back to 1969. Has the ball then been bouncing up and down the room since then? You should have seen it when you threw your “new” ball, then!
Or the same ball cannot exist twice at the same time, then somehow the 1969 ball stopped existing when the now-ball got created. And you will never get your ball back because it disappeared in the past.
Or the universe fixes itself, then when your ball disappeared in the time portal, the 1969 version somehow reappeared in the room, but that is the paradox that needs the most “fixing”, so I don’t prefer it
Oh ok, no problem. I can switch to a model that doesn’t shy away from paradoxes and contradictions.
Next, you would need to address what to do with those problems. Can we allow the same thing to exist in two places simultaneously? How about information and items without any origin? Does it matter if the model isn’t consistent or doesn’t make sense? If so, it’s going to be a very flexible tool when writing a scifi story.