Check if all fields in a specific column are uniquie

Hi,

I have a spreadsheet which has a column called "Reference".

I want to do a simple check that checks to make sure all references are unique e,g,

Reference
1

2

3

And to stop the workflow and display an error/message which says which reference is duplicated.

E.g. if the below was inputted:

Reference

2

1

2

3

1 

it would stop the work flow and say

"Reference 1 is duplicated"

"Reference 2 is duplicated"