WMI pulls current user even if never logged in - powershell

Question

I am new to powershell, and I am attempting to create a CSV from an excel file for comparison sake to see who is currently logged into a computer. However, I'm running into an odd issue where sometimes the script will pull the same user multiple times even if they've never logged into a computer. Here is my full code. I know there is a lot of optimization that can be done (and parts that need to be removed, those should be noted). I assume I've misused Get-WMIObject or something similar, can anyone see why it would pull that information like it is?
$csvRunFile = "test.csv"
$output = "Results_$(Get-Date -format yyyy.MM.dd).csv"
#Import the created csv.
$csv = import-csv $CsvRunFile
$results = foreach($csv_line in $csv) {
$ctag = $csv_line.ctag
$test_ping = test-connection $ctag -Count 1 -Quiet
#If the computer is pingable (IE: Online)
switch ($test_ping) {
$true {
#Pull the actual logged in user.
$Username = (Get-WmiObject -ComputerName $ctag -Class Win32_ComputerSystem).Username.Split("\\")[1]
#If the last modified folder is 'public' put an error, otherwise pull the username's information from AD.
#This was from when it pulled from the \User folder rather than the last log in, this is probably removeable.
if ($Username -eq "Public") {
$ADName = "No User"
} else {
$ADName = Get-ADUser -Identity $Username
$ADName = $ADName.Name
} #end If
}#end Switch:True
#Show there was an error when pinging the computer.
$false {$ADName = "ERROR"}
}#end Switch
#write the results the new output CSV.
$result = [PSCustomObject]#{
CTAG = $ctag
Username = $ADName
}#end PSCustom Object
$result
} #end foreach
#Turn the .txt into a CSV so it can be manually compared to the list in the original excel file.
$results | Export-Csv -path $output

Answer 1

There might be a leftover-value in $UserName for some reason or $ADName is still it's old value because you tried to run Get-ADUser -Identity $null when there was no user logged on (WMI returns $null when there's no user logged in).
I also changed your ping-test from a switch to an if-test just to cleanup the code. I've never seen Public being returned, but I left it as it doesn't really hurt either.
Try:
#If the computer is pingable (IE: Online)
if($test_ping) {
#Clear username var just to be safe
$Username = $null
#Pull the actual logged in user.
$Username = (Get-WmiObject -ComputerName $ctag -Class Win32_ComputerSystem).Username | ? { $_ } | % { $_.Split("\\")[1] }
#If the last modified folder is 'public' put an error, otherwise pull the username's information from AD.
#This was from when it pulled from the \User folder rather than the last log in, this is probably removeable.
if (($Username -eq "Public") -or ($Username -eq $null)) {
$ADName = "No User"
} else {
$ADName = Get-ADUser -Identity $Username
$ADName = $ADName.Name
} #end If username public
} else { $ADName = "ERROR"}