How do you remove duplicate entries in 2 different columns?

Okay, here’s what the problem is.

I have 2 columns with a number of URLs.
I need a function to highlight duplicate items in both columns so I can identify and superimpose the data.

The major issue is that there are nearly 20000 items in both rows :9

And I forgot to mention, I’m using Google Sheets.

Note: I figured out how to do this in a row using conditional formatting and the =countif(A:A,A1)>1 function, but how do I do it for two columns, let’s say – A and G.