Identify duplicate groups of cell in column

I have a large amount of list data entered on a sheet and need to know if any duplications have occurred. Is there any way to identify that groups of cells are unique to the sheet? This could be single or multiple occurrences of a string e.g. "town names". A formula would be preferable as opposed to vba. Possibly a conditional format?

e.g.:
A
Town name
Basildon
Basildon
Basildon
Bedford
Bedford
Gourock
Swansea
Swansea
Bedford <<< need warning of duplication here.

I can find many instances on how to identify duplicate single entries but not when grouped like this.

Thanks TMS, that works beautifully!
Marked as solved.