In Nichijou there is a scene where a random character says,
The coffee isn't very coffee coffee
Is this a pun? What's the meaning of these sentences? Even her friend asks too.
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There's no pun. She is using "coffee-coffee" as a made-up adjective to describe that the coffee she is drinking doesn't have properties of coffee, and as you can tell from her friend's response, it's not standard Japanese (she could've just said something like "This doesn't taste like coffee."). The subtitle uses the phrase as-is, but something more English could be something like "This isn't very coffee."
The gag of this skit is that Girl B drinks and agrees with the fictional word, despite no description of what it means. I feel this isn't supposed to be funny, but more closer to something the viewers can agree to, like when eating out with friends and trying to describe what you just ate/drank, but cannot find the right words for it (thus just saying something like "coffee-coffee").
Jimmy's answer is good, but I want to offer one clarification.
To an English speaker, "coffee-coffee" sounds an awful lot like contrastive focus reduplication. "this is coffee, but it's not COFFEE-coffee" might mean "this beverage has some but not all of the essential properties of coffee". However: Japanese doesn't have contrastive focus reduplication! That's not what's actually being said here.
Reduplication in Japanese is used primarily for mimetics (onomatopoeia, basically). Unlike with English's contrastive focus reduplication, you can't really apply "mimetic reduplication" to arbitrary Japanese nouns, so "kōhī-kōhī" (which is sort of a weird adjective-ified form of "coffee") sounds stranger to the Japanese ear than "COFFEE-coffee" sounds to the English ear. If I were to attempt an analogy, this might be somewhat like forming a strange derived adjective from coffee, as in "this coffee isn't very coffile".