Compare and highlight column data across two worksheets

Guys,

New User Here... I am at your doorstep because I need some quick help.

I have two worksheets in a single file. Each of these worksheets contains a column of numbers that both represent invoice numbers.

What I am trying to accomplish is to highlight numbers if they exist on both lists. THE HIGHLIGHT ONLY NEEDS TO HAPPEN ON 1 OF THE LISTS. It does not have to be a graphic highlight... It could be another column called 'On Both Lists" with a possible YES/NO answer. Each list has about 5000 lines so creating a formula or automated process is critical.

Note: each individual list can have duplicate invoice numbers (That is correct in the data). What I need to know is if a number exists on both lists.

PLEASE HELP AND THANKS IN ADVANCE FOR YOUR TIME AND SUPPOER.

Brian