How to output duplicated rows

Question

I have the following data:

x1  x2  x3  x4
34  14  45  53 
2   8   18  17
34  14  45  20
19  78  21  48 
2   8   18  5

In rows 1 and 3; and

Answer 1

Learned this one the other day. You won't need to re-order the output.

s <- split(dat, do.call(paste, dat[-4]))
Reduce(rbind, Filter(function(x) nrow(x) > 1, s))
#   x1 x2 x3 x4
# 2  2  8 18 17
# 5  2  8 18  5
# 1 34 14 45 53
# 3 34 14 45 20