R: show ALL rows with duplicated elements in a column [duplicate]

Question

Does a function like this exist in any package?

isdup <- function (x) duplicated (x) | duplicated (x, fromLast = TRUE)

My intention is to use it with dplyr to display all rows with duplicated values in a given column. I need the first occurrence of the duplicated element to be shown as well.

In this data.frame for instance

dat <- as.data.frame (list (l = c ("A", "A", "B", "C"), n = 1:4))
dat

> dat
  l n
1 A 1
2 A 2
3 B 3
4 C 4

I would like to display the rows where column l is duplicated ie. those with an A value doing:

library (dplyr)
dat %>% filter (isdup (l))

returns

  l n
1 A 1
2 A 2

Answer 1

dat %>% group_by(l) %>% filter(n() > 1)

I don't know if it exists in any package, but since you can implement it easily, I'd say just go ahead and implement it yourself.