Add something when you enter them so they differ from each other.
If you have 10 John Smith in a column how would excel know they are
different,
AFAIK you have to add a number (or another character) to the cell itself
or use a help column with unique keys for each person..
For instance assume you have 10 different John Smith in A1:A10, insert a new
column B and number it like 1,2,3 etc. then assume the values are in C1:C10,
now use
=SUMPRODUCT((A1:A10="John Smith")*(B1:B10=2),C1:C10)
of course for better usability you would change the hard coded name in the
formula to a cell reference so you can change it easier..