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

An alternative using ave:

dat[ave(dat[,1], dat[-4], FUN=length) > 1,]

#  x1 x2 x3 x4
#1 34 14 45 53
#2  2  8 18 17
#3 34 14 45 20
#5  2  8 18  5