Adding Multiple Results Using Vlookup

[hawong]

I am using Vlookup to search for staff members who have worked overtime, and return the hours worked in a summary sheet. The excel sheet is divided into months and I have created a summary sheet which has the Vlookup formula which is set up to search in the various months for the staff name and record the hours work in the summary sheet. My problem is:

Sometimes the staff name occurs more than once e.g. Andre Stevens occurs 3x in March. However Vlookup only returns one of the results. How do I get it to add all the results and return the sum in the Summary column?