Count number of duplicates value within groups and subgroups in R [duplicate]

I have a dataframe:

COL1 COL2  COL3 
A    Canis lup
A    Canis lupu
A    Canis lupus 
B    Canis lupus 
C    Canis rattus
C    Canis lupys
A    Lottus po
B    Lottus pi
B    Lottus pe 

and I would like to count for each COL1,COL2, the number of duplicate COL1 values and get:

COL1 COL2  COL3    Ndup 
A    Canis lup     3
A    Canis lupu    3
A    Canis lupus   3
B    Canis lupus   1
C    Canis rattus  2
C    Canis lupys   2
A    Lottus po     1
B    Lottus pi     2
B    Lottus pe     2

Here is the dput format if it can helps:

structure(list(COL1 = c("A", "A", "A", "B", "C", "C", "A", "B", 
"B"), COL2 = c("Canis", "Canis", "Canis", "Canis", "Canis", "Canis", 
"Lottus", "Lottus", "Lottus"), COL3 = c("lup", "lupu", "lupus", 
"lupus", "rattus", "lupys", "po", "pi", "pe")), row.names = c(NA, 
-9L), class = "data.frame")

Note : COL3 is just here to illustrate that I have other columns in the dataframe.

We can just easily use dplyr::add_count.

library(dplyr)

df %>%
  add_count(COL1, COL2, name = "Ndup")
#> # A tibble: 9 × 4
#> # Groups:   COL1, COL2 [5]
#>   COL1  COL2   COL3    Ndup
#>   <chr> <chr>  <chr>  <int>
#> 1 A     Canis  lup        3
#> 2 A     Canis  lupu       3
#> 3 A     Canis  lupus      3
#> 4 B     Canis  lupus      1
#> 5 C     Canis  rattus     2
#> 6 C     Canis  lupys      2
#> 7 A     Lottus po         1
#> 8 B     Lottus pi         2
#> 9 B     Lottus pe         2