This Script parses through all computers found in the domain. using the Object Class of Computer. (regardless of OU) : the problem I'm having is that the script outputs a loop index in one of the forgoing loops and I'm not sure how to prevent it.

Get-ADComputersPropertiesAndIPAddresses

function Get-ADComputersPropertiesAndIPAddresses {
    [CmdletBinding(DefaultParameterSetName = 'Identification', 
        SupportsShouldProcess = $true, 
        PositionalBinding = $false,
        HelpUri = 'https://www.github.com/skyhoshi/powershell',
        ConfirmImpact = 'Medium')]
    [Alias()]
    [OutputType([String])]
    Param
    (
        # Param1 help description
        [Parameter(Mandatory = $true, 
            ValueFromPipeline = $true,
            ValueFromPipelineByPropertyName = $true, 
            ValueFromRemainingArguments = $false, 
            Position = 0,
            ParameterSetName = 'Identification')]
        [ValidateNotNull()]
        [ValidateNotNullOrEmpty()]
        # [ValidateCount(0, 5)]
        # [ValidateSet("sun", "moon", "earth")]
        [Alias("AD Server Name or IP Address")]        
        $HostName
    )

    Begin {
        if ([System.io.File]::Exists("$env:USERPROFILE\desktop\ComputerActiveDirectoryReport-Prelim.csv")) {
            Remove-Item "$env:USERPROFILE\desktop\ComputerActiveDirectoryReport-Prelim.csv" -Force
        }
    }
    Process {
        if ($pscmdlet.ShouldProcess("Target", "Operation")) {
            ########################################################################################
            $AdHostSearch = [adsisearcher]("WinNt://$hostname")
            $AdHostSearch.Filter = "objectclass=computer";
            $searchResults = $null
            $searchResults = $AdHostSearch.FindAll() 
            ########################################################################################
            $HostNameList = New-Object -TypeName System.Collections.ArrayList;
            #$AdComputerPropertiesList = New-Object -TypeName System.Collections.ArrayList;
            foreach ($result in $searchResults) {
                #Write-Host $result.Properties
                foreach ($unTypedProperty in $result.Properties) {
                    #Write-Host "UnTyped Properties" -ForegroundColor yellow;
                    foreach ($prop in $unTypedProperty) {
                        $AdComputerName = $prop["cn"];
                        #Write-Host -ForegroundColor Green "Properties for computer $AdComputerName";
                        $HostNameList.Add($AdComputerName);
                    }
                }
            }
            #$AdComputerPropertiesList | Select-Object * | Export-Csv "$env:USERPROFILE\desktop\ComputerActiveDirectoryReport-Prelim.csv" -NoTypeInformation -Force

            ForEach ($hostn in $hostNameList) {
                #Write-Host $hostname
                try {
                    $hostEntry = [System.Net.Dns]::GetHostByName($hostn)
                    $hostEntry;
                }
                catch {
                    Write-Host "$hostn :: $_" -ForegroundColor Yellow 
                }
            }
        }
    }
    End {
    }
}

Get-ADComputersPropertiesAndIPAddresses -HostName "[Insert Your Local Domain controller host name here]" #HostName = Name of Domain Controller

Isolated Loop with issue

$hostname = "[Insert Your Local Domain controller host name here]"
########################################################################################
$AdHostSearch = [adsisearcher]("WinNt://$hostname")
$AdHostSearch.Filter = "objectclass=computer";
$searchResults = $null
$searchResults = $AdHostSearch.FindAll() 
########################################################################################
$HostNameList = New-Object -TypeName System.Collections.ArrayList;
#$AdComputerPropertiesList = New-Object -TypeName System.Collections.ArrayList;
$searchResultsCount = $searchResults.Count;
Write-Host "Search Results Count :  $searchResultsCount";
foreach ($result in $searchResults) {
    $resultPropertiesCount = $result.Properties.Count;
    Write-Host "Result Properties Count: $resultPropertiesCount";    
    #Write-Host "UnTyped Properties" -ForegroundColor yellow;
    foreach ($prop in $result.Properties) {
        $AdComputerName = $prop["cn"];
        #Write-Host -ForegroundColor Green "Properties for computer $AdComputerName";
        $HostNameList.Add($AdComputerName);
    }    
}

I think that the problem lies in the third loop, that is a secondary loop of the same information from the data duplicated from the second loop.
Loop 1 : line 41
Loop 2 : line 43
Loop 3 : line 45

Loop 2 and Loop 3 data output is equal in that their are two references to the same information

But WHY is it outputting the following

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

HostName                  Aliases AddressList
--------                  ------- -----------
REMOVED FOR SECURITY REASONS 

I added the second file to isolate and demonstrate the problem code.

A Friend on Slack helped (Thanks @coiscir : Twitter handle) with the answer :

$HostNameList.Add($AdComputerName); Returns an int. 

This can be updated to the following

$HostNameList.Add($AdComputerName) | Out-Null; Sends the output to Null.