This can be done by using SUMPRODUCT
as well. Update the ranges as you see fit
=SUMPRODUCT(($A$2:$A$7=A2)*($B$2:$B$7=B2)*$C$2:$C$7)
A2:A7 = First name range
B2:B7 = Last Name Range
C2:C7 = Numbers Range
This will find all the names with the same first and last name and sum the numbers in your numbers column