If I am reading you correctly,I dont think you want a sumif function
put 0 or some other staring number in D1
I think what you want in D2 is
=D1+if(A2=A1,c2,0)
and copy down

another factor could be if either what looks like matches in Column A are
not really matches
for two cells which look identical A3 and A4?
in a cell enter
=A3=A4 if the answer is not true they are not identical

additionally column C could be text rather than numeric
if this is the case try

=D1+if(A2=A1,value(trim(c2)),0)


"Easydoesit" wrote:

I am getting a 0 as my result, consistently. I have three columns. If
$A$n=$A$(n-1), I want to add the value in $C$(n-1) to $C$n. I incremented
the SUMIF value by extending the formula from D1 all the way down.

What might keep the function from working properly?