what is the correct way to force a copy of an object?

Dear R-devs,

I'm working on a package where we have a function that modifies an
Object via .Call  to a C function. Unfortunately in some situations this
counterintuitive modifies a previously made copy of the object passed to
the function.  For example, if I first make an assignment to "copy" the
object,

b<-a

and then modify 'a' , the value of 'b' will be modified as well.  I
understand that this is probably due to lazy evaluation--the copy of 'a'
is not made until the 'a' or 'b' instance is further modified. Peeking
at 'a' and 'b' using .Internal(inspect(a)) seems to confirm this.  But
for some reason our function is not triggering the copy.

We have been able to kludge it to force a copy and behave as expected by
making a slight modification to the object before passing it to .Call:

modificationFun <- function(x){
    x$ForceCopy<-NULL
    x<-.Call("do_the_mod_in_c_R", package='mypackage')
    invisible(x)
}

What is the correct way to force the copy to occur to avoid
inadvertently modifying both instances of the object?  Neither force()
or eval() seem to do this.

Would it be faster to call duplicate(x) inside the C function instead of
forcing the copy at the R level?

thanks for your help,
   -skye