any Excel pros here?

I am going through different ahrefs reports and copy/pasting any backlinks that I like to investigate into an Excel sheet. When I copy/paste sometimes the backlink already exists in the excel sheet because I had already found it from another competitor's website, so I end up with a lot of duplicates.

so for examples,

Forbes.com
Engadget.com
Articlebase.com
Forbes.com

see? Forbes appears twice.

Is it possible to set up excel so as soon as I paste the duplicate domain, excel somehow indicates that this entry already exists?

No it's not that easy. There are tutorials online for how to do it.
https://www.google.com/search?q=excel+remove+duplicate+rows

Personally I find it too tedious. I prefer http://textmechanic.com/text-tools/basic-text-tools/remove-duplicate-lines/ and it's a quick snip job.

Hi, you can just google it! "don't paste duplicates in excel" and the first result is your solution. Regards!