Mr. Low:

If you're still looking for a formula, I would modify Jim's answer as
follows:
=IF(A2=A3, "", SUMIF($A$2:A2, A2, $C$2:C2))

- David

Jim Thomlinson wrote:
=IF(A2=A3, "", SUMIF($A$2:$A$24, A2, $C$2:$C$24))