You can clone an NFT by forking the blockchain it's on, or making another one with the same data. People who respect NFT ownership won't be able to tell without research which one is real.
If I make a counterfeit Picasso and sell it to someone who doesn't know any better, that doesn't mean the original is now fungible, it just means I duped someone.
Ha, well, I'd argue that the generalized version of what I said applies as well. Forking a currency changes nothing about the properties of the original currency because we're no longer talking about the original currency. That is not to say there aren't real world implications of that happening, but that's nothing to do with the intrinsic properties of a currency.
A NFT isn't art or currency. At most, it's a record of ownership. When two ownership records exists, the value of the original drop steeply when the copy is accepted widely-enough.
I don't disagree, but I think that's still orthogonal to the discussion and, considering NFTs are non-fungible by definition, it's moot anyway.
But let's pretend we're talking about cryptocurrency, which does claim to be fungible. Bitcoin forked and so now you have BTC and BCH. One is more accepted, and the other lost value. But regardless of the relationship between the two, each individual currency is still fungible because you can still exchange 1 BTC for 1 BTC. Fungibility is an intrinsic property of a single currency, it says nothing about it's relationship to other currencies or it's place in the economy.