Question (spoilers)

It was the most confusing part in an already complicated plot, but makes sense once you re-watch.

Let's say that Anna originally goes missing in April (2024). Ellie goes looking for her, enters the time tunnel and sees Anna. But it is now (roughly) a month before the day she goes looking. Celeste is shown talking to the cops on the phone, saying Anna had followed her big sister alone a month ago and that's why when April Ellie sees Anna stuck in the boat, Anna still has her tooth intact and asks Ellie why she behaves that way (referring to a fight that April Ellie had had with her mom a month earlier before storming off, which from Anna's perspective, happened only a few hours earlier).

April Ellie then escorts (March) Anna back to their home (in March) and leaves. She goes back into the tunnel, exits in 2005, realizes the truth about Anna growing up in the past and returns to her current timeline (2024), finally letting her mom know Anna lived in the past.

Ellie is the only one who exited the time tunnel 1 day (or a few hours) after the first time she went through it, which is why she sees her double. The other 2 people who traveled through the tunnel end up in a time when they didn't exist, which is why they don't see a double of themselves - Anna, who gets taken back to 1952 where she originally never existed, and Paris, who travels to the future (from 2004 to 2024), where there is no living version of him, since he dies on the same day. Ellie also runs into her toddler version in 2005, when she encounters her mom after trying to grab the necklace from her car.

Good movie indeed, caught me by surprise and reminded me a bit of 'Where the Crawdads sing', in that it was well done, with a solid cast, no nauseating political BS and entertaining without a huge budget or CGI overload.