The joke is that you don’t want to see the other person, so they say ‘see you later!’ and they say 'not if i see you first HA HA! (i do not wish to associate with you you, so i will hide / avoid you / run away if i see you first (I do really want to see you, this is all a hilarious ‘joke’ lol))
No, if you’re both in a supermarket or something browsing then I see you first when I look up from browsing. Its winning at seeing someone in a spontaneous situation, you don’t pre-arange to hide from each other, that would be weird, or are you all arranging hiding competitions and I’m just not invited?
It just never occurred to me that you might talk to someone, then say “yeah we’ll meet up again sometime” only for the other person to be thinking “nah I’m gonna hide” so duplicitous. So mean. Just wouldn’t even enter my head. Even in jest.
I’d also like to caveat this with I don’t really think it means either, it’s just a terrible bit of dad comedy/shit word play innit, but it definitely doesn’t mean hiding
‘I’m going to see that new film tonight!’
‘Not if I see it first!!!’
‘Well, no because I’ll still see the film won’t I?’
‘Not if I see it first you won’t!!!’
‘Fuck off m8’
I feel like it’s only ever been used seriously in the context of 1930s private eye film noir stuff though I mean yeah, people use it because it became a staple and in the context of mates it’s weirdly jokey, but it would be the sort of sign off from the likes of Philip Marlowe to some detective who’s got his eye on Marlowe because, obviously, he thinks he either did the murders or knows more than he’s saying.
E.g.
Detective: See you later.
Marlowe: Not if I see you first! grins
And obviously it’s good natured, the detective has no evidence, he can’t hold Marlowe on anything but he’s suspicious and he’s right to be. It’s a whole sparring thing via pleasant-seeming dialogue between two guys who have professional respect for each other at some level.
It’s just that it’s a cliché and people say them even though they don’t really know what they mean, much like ‘exception that proves the rule’ is sort of used instinctively without anyone taking the time to interrogate its etymology or even if it really applies in the current situation.
[I have to say, though, anxiety over the idea of people trying to avoid you isn’t something I ever considered before. I mean obviously I just accept that most people want to avoid me so I guess it just seems a natural sort of thing to say.]