Autofill if only one viable option

I am helping a someone who has been running a payroll to update the process with Excel. I'm trying to do the following:

-There is a master employee list in it's own Sheet

-In the main worksheet I want the First Name to auto-populate if there is only one person with the Last Name I've entered, otherwise allow me to write the First Name.


I'm trying to streamline the process so it takes as little time as possible. Any help would be appreciated. Let me know if I need to offer up more info.