Easily add invoice numbers when there are multiple rows with same name

Hi,
I was wondering if there is an easy way to do this job I have to do every month. I create all the information on Excel then upload into our invoicing program. Down column A are all the clients names and there are multiple rows for each person for each date there is a job entry. In the next column I need to enter the invoice number however the invoice number needs to be the same for each name so when I upload it uploads as one invoice per client name. I wanted to know if there is easy way to create a formula to copy down rather than the slow manual way. I have attached an example which I hope shows what I am trying to explain.
Thank you in advance if anyone can assist.

Thank you so much! This is amazing and has saved me so much time!