row_number()

グループに分けて番号を振る

元データ
> d = tibble(name = c("A", "B", "C", "A", "A", "C" ), value = c(1,4,2,3,5,5))

方法１

> d |> group_by(name) |> mutate(add = 1, dup = cumsum(add)) |> select(-add)
# A tibble: 6 × 3
# Groups:   name [3]
  name  value   dup
  <chr> <dbl> <dbl>
1 A         1     1
2 B         4     1
3 C         2     1
4 A         3     2
5 A         5     3
6 C         5     2

方法２

> d |> group_by(name) |> mutate(dup = row_number(name))
# A tibble: 6 × 3
# Groups:   name [3]
  name  value   dup
  <chr> <dbl> <int>
1 A         1     1
2 B         4     1
3 C         2     1
4 A         3     2
5 A         5     3
6 C         5     2