If you want a single cell to contain the sum from column C of all
corresponding matches between A and B in rows 1 to 100 (for example),
you can use this formula:

=SUM(IF((A1:A100=B1:B100),C1:C100,0))

It is an array formula, which means that once you have typed it in (or
subsequently edit it) you have to use CTRL-SHIFT-ENTER instead of just
ENTER. If you do this correctly then Excel will wrap curly braces { }
around the formula - you must not type these yourself.

Hope this helps.

Pete