Identifying Duplication in a Column - New Form Submissions

Hello, 

I am trying to see if there is a formula I could use in a hidden column, with a checkbox, where when a new form is submitted (and a new line is then entered onto a sheet) it would alert me if there was a duplication in a certain column. 

Example: We enter new sales orders into Smartsheets using a form. If an order number is submitted twice, the box on each line where the instance occurs would be checked and then I could set up conditional formatting to highlight the two lines. I can get this to work once, but not when a new line is added to the sheet. 

Hopefully this makes sense, appreciate any help!

Thank you, 

Allison