Function arguments and copying objects

I have a function that, via .Call, modifies its argument, so I first
create a backup copy of the object I want to pass, make a copy of it,
and pass the copy to the function. However, my function also modifies
the backup copy. I guess this has to do with R not duplicating objects
until they are modified. Where can I read more about that? And/or how
can I solve my problem, i.e. save a copy of the argument?

Here is an example of what I mean:

mod.back$m0 <- rnorm(2)
mod.back

$m0
[1] -0.03431583 -1.66513535

...

mod <- mod.back
dlmLLsvd(ip,mod)
mod.back

$m0
[1] 3.360737 3.266844

...


Thanks in advance,
Giovanni

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