This script uses the Active Directory PowerShell module to locate duplicate proxyaddresses throughout your forest. The script must be run from a computer that have the AD PowerShell installed, and can reach all PDCEmulators in all domains in your forest.

Import-Module ActiveDirectory

# Create hashmap for proxyaddresses
$proxyaddresses = @{}

# For each domain in the forest
Get-ADForest | Select-Object -ExpandProperty Domains | Get-ADDomain | foreach {
    Write-Output ("Parsing domain {0} by contacting {1}" -f $_.Name, $_.PDCEmulator)
    # Get all AD objects that have proxyaddresses
    Get-ADObject -Filter {proxyaddresses -like "*"} -Properties proxyaddresses -Server $_.PDCEmulator | foreach {
        $_.proxyAddresses | foreach { $proxyaddresses[$_] += 1}
    }
}

Write-Output "Done, looking for duplicates"
$duplicates = $proxyaddresses.Keys | where{$proxyaddresses[$_] -gt 1}


# Output proxyaddresses that are duplicates
if($duplicates) {
    Write-Output "The following proxyaddresses was found multiple times"
    $duplicates # | Out-Gridview # Remove first hash-sign in order to get an "Excel"-view. Needs PowerShell ISE.
} else {
    Write-Output "No duplicates found"
}