R - create random numbers

I have huge dataframe like this:

df <- read.table(text="
             id       date
1            1        2016-12-01 
2            2        2016-12-02 
3            4        2017-01-03 
4            6        2016-11-04 
5            7        2017-11-05 
6            9        2017-12-06", header=TRUE)

I generate randomly 1 or 0 for each id. I'm doing it with this code.

set.seed(5)

df %>%
  arrange(id) %>% 
  mutate(
    rn = runif(id),
    discount = if_else(rn < 0.5, 0, 1)
  ) 

It works perfectly until I add new rows to my dataframe. Then are my random numbers different.

But what I need is not just generate random number for each id, but that number has to remain same even if new rows are added.

That means:

             id       date           discount
1            1        2016-12-01            1
2            2        2016-12-02            0
3            4        2017-01-03            0
4            6        2016-11-04            1
5            7        2017-11-05            1
6            9        2017-12-06            1

When new rows are added

             id       date           discount
1            1        2016-12-01            1
2            2        2016-12-02            0
3            4        2017-01-03            0
4            6        2016-11-04            1
5            7        2017-11-05            1
6            9        2017-12-06            1
7            12       2017-12-06            0
8            13       2017-12-06            1