"Double dipping" is just bullshit dealer-speak for "yo, buy my shit, it's better than everyone else's". Depending on the dealer (ie, some dumb kid), they might even believe it because their supplier told them the same thing. But either way, avoid dealers like this. "Double dipping" acid is not a thing.
When I was a kid, it was 7 times, and it was always worded "clinically insane". But that urban myth survived. I THINK that it's finally gone the way of the dinosaurs, what with all the talk of Silicon Valley tech geniuses microdosing at work to aid in their creativity.
The funniest one for me was always that in any group of people, someone knew someone whose friend's brother's cousin took too much acid and now believes they're a glass of orange juice, and refuses to leave their room because they're afraid of being spilled. I've had a shocking number of people tell me that one and seem to be entirely serious.
Oh, and lest I ever forget... "red rock opium". Anyone else experience this myth as a kid? In high school, literally everybody was always on the lookout for anyone selling "red rock opium". The prevailing idea was that it was opium, except in the form of fragrant red chunks. It supposed had to be smoked with weed to work, but it made the weed way stronger (because after all, what is opium, really, but super-weed??). I paid $80 for a little tiny bag of it once, and was sure that I got much higher than I would have if I had just smoked my weed. Oh yeah, 90% sure.
In reality, it was dragon's blood incense, a specific variety, sold wherever you could get incense, pretty much. Totally inactive, but a tremendously successful scam-myth.