How to keep rows without duplicate in R

Question

I have a data frame with many duplicated rows:

1    3
1    2
2    5
2    4
2    5
3    7
3    7
3    7
3    7
3    7
3    8
3    7
3    7
4    2 
4    5 
4    2

I want to keep just rows that repeat once in data.frame:

1    3
1    2
2    4
3    8
4    5 

Would you please guide me?