Help with an excel formula

Hi All,

I'm trying to make a spreadsheet at work and attached is a basic version of what i'm trying to do. Mainly concentrating on tab "sheet 2" this will be a log sheet for recording invoice numbers paid against agency staff members.
Col A = staff ID number. I have used the data validation tool/ list so we can select any of the staff ID numbers from the first tab "sheet 1"in column A
Col B = Weekending dates (sundays) and as above I have currently created a list you can select from from the first tab using row 2
Col C = this automatically selects the surname to match the staff ID number from tab 1
Col D = we will type in the invoice number
Col E = the date we sent this to accounts payable to process and pay the invoice

Basically what I am trying to do is be able to create a list to log the invoices I approve which containing what the invoice number is, who the payment relates to and what week ending the payment covers.

What I'm stuck with is I need the spreadsheet to some how alert me if I am entering a staff ID number and a W/E date which has already been entered before. As with the attachment rows 7 and 8 on tab 2 both contain staff ID 1 (smith) with 2 invoice numbers containing the same w/e date. Is there some sorta way I can have a spreadsheet that will highlight any rows that have identical data in columns A and B?

Also not as important I was tring to find a way that on tab 1, could the green section automatically update itself with the invoice number using data from tab 2?

Any help will be well appreciated!!

cheers shane