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Juan Pablo Gallardo 

Replace the last two lines with below code to remove headers

$csvOutput | Select-Object C1,C2,C3,C4,C5,C6,C7,C8,C9 |
ConvertTo-Csv -NoTypeInformation | # Convert to CSV string data without the type metadata
Select-Object -Skip 1 | # Trim header row, leaving only data columns
Set-Content -Path "C:\Users\UserName\Documents\output.csv"

Complete script :

$filepath = 'C:\Users\UserName\Documents\input.csv';
$csvInput = Import-CSV $filepath -Header C1,C2,C3,C4,C5,C6,C7,C8,C9
$csvOutput = @();
$csvInput | Foreach-Object{ 
$currentRow = $_;
$existingRow = $csvOutput | Where-Object { $_.C4 -eq $currentRow.C4}
if($existingRow -ne $null) {
$existingRow.C8 = $existingRow.C8 + [int]$currentRow.C8; 
} else { 
$csvOutput += New-Object -TypeName PSCustomObject -Property @{
C1 = $currentRow.C1
C2 = $currentRow.C2
C3 = $currentRow.C3
C4 = $currentRow.C4
C5 = $currentRow.C5
C6 = $currentRow.C6
C7 = $currentRow.C7
C8 = [int]$currentRow.C8
C9 = $currentRow.C9
}
}
}

$csvOutput | Select-Object C1,C2,C3,C4,C5,C6,C7,C8,C9 |
ConvertTo-Csv -NoTypeInformation | # Convert to CSV string data without the type metadata
Select-Object -Skip 1 | # Trim header row, leaving only data columns
Set-Content -Path "C:\Users\UserName\Documents\output.csv"