Assign unique value for duplicated rows

Question

I want to assign the value for each duplicated row by ID in R

df <- data.frame(ID=c(1,1,1,2,2,2,2,2,3,3,4),
            Code = c("A","A","A","B","B","C","C","D","A","A","C"))
> df
   ID Code
1   1    A
2   1    A
3   1    A
4   2    B
5   2    B
6   2    C
7   2    C
8   2    D
9   3    A
10  3    A
11  4    C

I want the output like this,check duplicated by ID, then assign the second duplicate _1 and so on...

   ID Code Code_n
1   1    A      A
2   1    A    A_1
3   1    A    A_2
4   2    B      B
5   2    B    B_1
6   2    C      C
7   2    C    C_1
8   2    D      D
9   3    A      A
10  3    A    A_1
11  4    C      C

Answer 1

You can use make.unique from base R as follows,

with(df, ave(as.character(Code), ID, FUN = make.unique))
#[1] "A"   "A.1" "A.2" "B"   "B.1" "C"   "C.1" "D"   "A"   "A.1" "C"

Answer 2

Or using dplyr

library(dplyr)
df %>% 
    group_by(ID) %>% 
    mutate(Code_n = make.unique(as.character(Code)))