Orphaned shadow fields cause "a duplicate name ... was found" - PowerShell solution

Question

When you create a site column of TaxonomyField type, SharePoint creates a "shadow" hidden field with the same name plus a trailing underscore and number. When you delete the main taxonomy field, SharePoint does NOT delete the shadow field. If you
are creating custom fields programmatically (or by installing a feature), you will get an error complaining of a duplicate field name. You cannot resolve this through the UI, but PowerShell can help. After a day of pulling my hair, I wrote the following PS
script. Please comment and improve: I did some testing, but hardly rigorous.
# SiteColumnDuplicateKill
# Run in PowerShell IDE
################## Constant Declaration ##########################################
$siteUrl = "https://your.sitecollection.com"
<#
    This script removes site columns that have been orphaned when a custom site column
    of type TaxonomyFieldType (or TaxonomyFieldTypeMulti) has been deleted. When such a column
    is created, SP creates a "shadow" field with the same name appended with an underscore
    and a number. For example, if you create a TaxonomyFieldType named "Foo", SharePoint
    creates a hidden column called "Foo_0". If you delete the Foo column, SharePoint 
    *DOES NOT* delete the shadow column! If you create another metadata column called
    "Foo", SharePoint will create a new hidden column called "Foo_1", because "Foo_0" already
    exists. As long as you create columns with the SP user interface, this bit of poor
    housekeeping causes no problems. 
    BUT -- if you create a site column by installing a feature, either via a manual wsp install
    or via Visual Studio's deploy function, there is a problem. SharePoint will look at the
    metadata column name in your feature and check to see if there is an existing shadow column
    corresponding to that name. E.g. if your feature includes a column called "Foo", SharePoint
    will look for a hidden column called "Foo_0". If it finds it, SharePoint throws an error
    complaining of a duplicate field name. This is clearly a bug: it ought to behave like the
    UI does: increment the tail number until it gets a unique name. Better still, the bug
    shouldn't exist at all: SharePoint shouldn't keep those shadow columns. But the bug is there,
    and this script solves the problem. It finds orphaned columns and removes them.
    Run this script whenever you delete a custom metadata site column -- either via the UI or
    by de-activating and removing a feature or by using the Visual Studio Retract function.
    NOTE: internally, site columns are called "Fields". I use the terms interchangeably in 
    my comments.
#>
function MakeFieldRef($aFieldTitle) {
    # Creates a PS custom object with information about a shadow field
    # Shadow fields look like this: "My metadata field_0"
    $mainFieldTitle = ""
    # Extract the main field title (the part before the last _)
    $pieces = $aFieldTitle.split("_")
    for($i = 0; $i -lt $pieces.length - 1; $i++) {
        $mainFieldTitle += $pieces[$i] + "_"
    # Remove the extra trailing _
    $mainFieldTitle = $mainFieldTitle.Substring(0, $mainFieldTitle.Length-1)
    # Instantiate the custom object
    $properties = @{'FieldTitle' = $aFieldTitle;
                'RootTitle'=$mainFieldTitle;
                'Index'= 0 + $pieces[$pieces.length - 1]}
    $object = New-Object –TypeName PSObject –Prop $properties
    return $object
###################### Main Processing ##################
clear-host
Add-PSSnapin Microsoft.SharePoint.PowerShell -erroraction SilentlyContinue
$site = Get-SPSite $siteUrl
$web = $site.RootWeb
#Find all hidden fields that end in an underscore plus one or more digits
$shadowFields = $web.fields | ?{$_.Title -match "_+\d" -and $_.Hidden -eq $true} | Select Title
#$shadowFields
<#
Loop through these shadow fields. For each, determine if there is a corresponding "main" field.
If not, delete the shadow field, because it's not needed. E.g.:
    Shadow field: Foo_0
    If no field called "Foo", delete the shadow field
If there *is* a corresponding main field, we need to preserve the shadow field
that goes along with it. But what if there are several shadow fields? E.g.:
    Shadow fields: Bar_0, Bar_1, Bar_2
    If there is a field called "Bar", we must not delete Bar_2, because it 
    corresponds to that main field. But Bar_0 and Bar_1 are remnants of
    site columns that have been deleted, so we can get rid of them
#>
<# 
    Instantiate an array to hold the field ref objects. A FieldRef has properties FieldTitle, RootTitle and Index.
    RootTitle is the part of the field title preceding the last underscore, and Index
    is the integer value of the number following the last underscore. E.g.:
        shadow field title:  "Silly old Foo_1"
        FieldRef.FieldTitle:  Silly old Foo_1
        FieldRef.RootTitle:   Silly old Foo
        FieldRef.Index:       1
    We need this object to be able to sort the shadow fields in numerical order. Otherwise Foo_10 would sort before F00_9 
#>
$fieldRefs = @()
foreach($field in $shadowFields) {
    # Create a field ref object with properties FieldTitle, RootTitle and Index
    $fieldRef = MakeFieldRef $field.Title    
    $fieldRefs += $fieldRef
# Add a dummy entry to the end. Kluge to avoid the problem of 
# an array of a single element showing a length of nothing
$dummy = MakeFieldRef "zzzzzzDummy_999"
$fieldRefs += $dummy
# Sort the array of field refs
$fieldRefs = $fieldRefs | sort-object RootTitle, Index
<# Now go through the array of objects and determine if there is a corresponding main field#>
for($i=0; $i -lt $fieldRefs.length - 1; $i++){
    $mainField = $web.fields[$fieldRefs[$i].RootTitle]
    # Does this main field exist?
    if(!$mainField) {
        # No - kill this shadow field
        $web.fields.delete($fieldRefs[$i].FieldTitle)
        write-host "Deleted " $fieldRefs[$i].FieldTitle
    } else {
        # Yes - we will kill this field if the *next* field ref has the same RootTitle
        $j = $i + 1
        if ($j -lt $fieldRefs.length -and $fieldRefs[$j].RootTitle -eq $fieldRefs[$i].RootTitle) {
            # The next field ref has the same root title, so we can delete this one
            $web.fields.delete($fieldRefs[$i].FieldTitle)
            write-host "Deleted " $fieldRefs[$i].FieldTitle
Leigh Webber

Answer 1

So can we kill these ones when they are in a collecting state? I thought you could.