VB.Net - Comparing Two DataTables

Good Day Experts!

I have 2 lists of AccountNumbers.  I am looking for duplicate product numbers across all the accounts.  Here are my 2 lists and a little summary of what I am doing:

1     1
2     2
4     4    
5     5
6     6

I loop through first list comparing to all in the second list.  Problem I have is...say I find a duplicate in Account 1 and 2...then I also get the same record when on Account 2 and comparing to Account 1.  

Do you have any ideas on hos I can better do this compare to eliminate the extra records?

Thansk,
jimbo99999