!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