C (gcc), 14 13 bytes

f(n){n=n?:1;}

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C, 17 bytes

f(n){return!n+n;}

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C, 16 bytes

#define f(n)!n+n

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C (gcc), 14 13 bytes

f(n){n=n?:1;}

Thanks to @betseg for reminding me of the n?:1 trick in the comments of the other C answer!

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C, 17 bytes

f(n){return!n+n;}

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C, 16 bytes

#define f(n)!n+n

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C (gcc), 14 bytes

f(n){n=n?n:1;}

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C, 17 bytes

f(n){return!n+n;}

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C (gcc), 14 13 bytes

f(n){n=n?:1;}

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C, 17 bytes

f(n){return!n+n;}

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C, 16 bytes

#define f(n)!n+n

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