I figured it out finally! I was trying typing in the header name for the column in and out of quotations and in the last step you had changed the beginning from =IF(COUNTIF(A$1:A1,A2)=0 instead to =IF(COUNTIF(C$1:C1,C2)=0 so I got really confused. The following formula is what actually ended up working for me (Of course I had to change the column letters and row numbers to fit where my data started and where I wanted to pull it from but this is the equivalent to your example).
=IF(COUNTIF(A$1:A1,A2)=0,SUMIF(A,A2,C),””)

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