R, 33 bytes

(Or 26 bytes in R>=4.1 using ‘\’ instead of ‘function’)

function(x)sample(y<-c(0,x),1,,y)

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The 4th argument of R’s ‘sample’ function is ‘prob’=“a vector of probability weights for obtaining the elements of the vector being sampled”.

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R, 29 bytes

(Or 22 bytes in R>=4.1 using ‘\’ instead of ‘function’)

function(x)sample(rep(x,x),1)

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Only valid for integer input.

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R, 25 bytes

(Or 18 bytes in R>=4.1 using ‘\’ instead of ‘function’)

function(x)sample(x,1,,x)

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This simpler version of the first solution may unfortunately fail if the input vector has only 1 element (due to a ‘convenience’ feature of ‘sample’ which changes its behaviour in this case…).