Could we argue that "por si" translates literally to "for if" as well as the synonymous "in case"?
I don't think any native English speaker would ever say "This is for if I don't return" (the correct translation duolingo gives)
Native English speaker here! I've heard and said both "in case" and "for if." Think of it this way, someone asks you "What is this for?" and you respond, "This is for..." Adding "...if I don't return" to the answer is a perfectly natural way to speak.
Disagree, I've heard that quote plenty of times in movies and in person (at work, etc).
Around here, we'd say "This is in case I don't come back" (like if your boss gives you the pizza coupon for work or something) but "this is for if I don't return" is a very "movie" sentence to say.
Both translations work for me although I favor the less literal "in case". However my wife is a native Spanish speaker and I might change my preference when speaking to her in English just to eliminate any ambiguity about the meaning of "case."