Highlighting multiple duplicates based on two columns

Hi,
This is driving me insane as it seems so simple, but not for me!
I'm hoping my example sheet explains it all, but if not please do ask.

Any help is greatly appreciated.

Thank you

Select B2:B29 then go to

Conditional Format > Use a last option "Use a formula to determine which cell to format"


=COUNTIF($A$2:$A$5,$B2)

Thank you very much. Works perfectly. I appreciate your help