Finding Duplicate Entries

Hi all,

I have a file which contains Licence Numbers and Serial Numbers.

There are at least 1 and a max of 2 Serial numbers per licence and each Serial number is unique.

However, a user has mistakenly added existing Serial numbers to some Licences.

Q. How can I find all the Licences which have identical Serial Numbers?

I need to link this file to itself somehow to highlight any duplicates.

I looked through the existing threads and can only find 1 topic covering this subject and it has not been answered.

Any help would be appreciated.

Thanks, Sylvie