Duplicates in comparing two columns

I am have a great deal of trouble. I have to compare over 10000 numbers between two columns. I cannot find a formula that will compare the two rows instead of comparing the information in just that one row. My point is say i have 10,000 numbers in one column and 10,000 in another column. I need to compare the two columns and make sure there are no duplicates between them. Can you help?? I have beensearching through threads and i cant find anything that works!

ex:

Col:A Col:B
1 ---------7
2 ---------8
3 ---------9
4 --------12
5 --------2 *
2 --------3 *
3 --------15
4 --------1 *
5 ---------3 *
6 ---------13

how can i find these duplicates without looking through each one?