Match EXACT data

I have a list of about 500 names of customers on an Excel Spreadshet and each has a unique reference number. Each month I will be importing about 40 new customers from another Excel spreadsheet.

I need to check if the new customers are already on the list of 500 names and the best way is to check by the unique reference number.

Is there a way to match the new 40 with the full 500 in one go. I looked at the Excel help menu and found 'EXACT' but I could not understand it.

If this is the correct way, can someone explain step by step.

Many thanks