Trying to highlight duplicates

Hi there.

I am trying to come up with a formula that will allow me to highlight duplicate cells but only if it is a duplicate for a value in another column as well.

What I mean is: 

I want the values in column B to be highlighted if they are the second duplicate for the values in column A.

So the only cell that should be highlighted should be B6

I know you can create a 3rd column and concat column A & B into 1 unique value and then find duplicates there, but I'd prefer not to add columns

So the end result would look like this 

Any help would be appreciated!