R does have call-by-need semantics, like (an impure) Haskell, but that's not why replicate's second argument gets evaluated multiple times:

  replicate <- function (n, expr, simplify = TRUE) 
     sapply(integer(n), 
        eval.parent(
           substitute(function(...) expr)), 
      simplify = simplify)

The replicate function uses R's compute-on-the-language to construct a new function that has expr as its body.

A closure's argument will be evaluated at most once.