Why does duplicate() make deep copies?

Some experimentation with the below function should convince you that the
runtime of the bit inside sys.time is proportional to size*number*times. I
think it should only be proportional to number*times. The function is only
manipulating a list of references to vectors and not trying to make changes
to the vectors themselves.

overcopying <- function(size, number, times) {
 #Make a list of NUMBER vectors of SIZE, then reorder the list.  The
 #vectors themselves are never touched, only their references are
 #moved around.  If R implements "copy on write" correctly the
 #elapsed time should be ~number*times.
 L <- replicate(number, list(vector("numeric", size)), simplify=FALSE)
 system.time(for (i in 1:times) {
   L[sample(number)] <- L
 })
}

I see that duplicate.c makes a recursive copy of each element when it
encounters a VECSXP or a LISTSXP, which it seems there should be no need
for (it should be sufficient to make a shallow copy and ensure NAMED is set
on the elements.)

Why is R making apparently unnecessary deep copies?