The script from my blog post at http://blog.codeassassin.com/2007/10/13/find-duplicate-files-with-powershell/

Get-DuplicateItems.ps1

param ([string] $Path = (Get-Location))

function Get-MD5 (
    [System.IO.FileInfo] 
    $file = $(throw 'Usage: Get-MD5 [System.IO.FileInfo]')
) {
    # This Get-MD5 function sourced from:
    # http://blogs.msdn.com/powershell/archive/2006/04/25/583225.aspx
    $stream = $null
    $hashAlgorithm = New-Object -TypeName System.Security.Cryptography.MD5CryptoServiceProvider
    $stream = $file.OpenRead()
    $hashByteArray = $hashAlgorithm.ComputeHash($stream)
    $stream.Close()

    ## We have to be sure that we close the file stream if any exceptions are thrown.
    trap {
        if ($stream -ne $null) { $stream.Close() }
        break
    }

    return [string]$hashByteArray
}

$fileGroups = Get-ChildItem -Path $Path -Recurse |
    Where-Object { -not $_.PSIsContainer } | 
    Where-Object { $_.Length -gt 0 } |
    Group-Object Length |
    Where-Object { $_.Count -gt 1 }

foreach ($fileGroup in $fileGroups) {
    foreach ($file in $fileGroup.Group) {
        Add-Member -MemberType NoteProperty -Name ContentHash -Value (Get-MD5 $file) -InputObject $file
    }

    $fileGroup.Group |
        Group-Object ContentHash |
        Where-Object { $_.Count -gt 1 }
}

Fixed type reference and filtered out directories.