Delete duplicate fields

Find all duplicated fields in a list under all content types.

 Clear-Host  
 Add-PSSnapin microsoft.sharepoint.powershell -ErrorAction SilentlyContinue  
 [System.Reflection.Assembly]::Load("Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")   
 $web=Get-SPWeb "http://site:1234"  
 function cleanFieldLinks($listName){  
   $list = $web.GetList($listName)  
      ForEach($ctype in $list.ContentTypes){  
      write-host $ctype.Name": "$ctype.ID  
           $ct = $ctype  
           $flDub = $ct.FieldLinks | group-object DisplayName | where { $_.Count -gt 1 }  
           foreach($fl in $flDub) {  
                $skipFirst = $fl.Group | select-object -skip 1  
                foreach($flDel in $skipFirst){  
                     $ct.FieldLinks.Delete($flDel.Id)  
                }  
           }    
           $ct.Update()  
      }  
 }  
 cleanFieldLinks("/Library")  
 $web.Dispose()  

Source