Find duplicated elements with dplyr

I guess you could use filter for this purpose:

mtcars %>% 
  group_by(carb) %>% 
  filter(n()>1)

Small example (note that I added summarize() to prove that the resulting data set does not contain rows with duplicate ‘carb’. I used ‘carb’ instead of ‘cyl’ because ‘carb’ has unique values whereas ‘cyl’ does not):

mtcars %>% group_by(carb) %>% summarize(n=n())
#Source: local data frame [6 x 2]
#
#  carb  n
#1    1  7
#2    2 10
#3    3  3
#4    4 10
#5    6  1
#6    8  1

mtcars %>% group_by(carb) %>% filter(n()>1) %>% summarize(n=n())
#Source: local data frame [4 x 2]
#
#  carb  n
#1    1  7
#2    2 10
#3    3  3
#4    4 10