A Web Part with this ID has already been added to this page.

Within a SharePoint site that we’ve upgraded, we have an error showing when users navigate to a page :

Using some PowerShell, I was able to list the specific webparts on the page :

$web = Get-SPWeb http://intranet/Services/Finance 
$list = $web.Lists["Pages"] 
  
#get the page (listitem) 
$item = $list.Items[0] 
 
#get the webpart manager 
$wpm = $item.File.GetLimitedWebPartManager('Shared') 
 
$wpm.WebParts | select id, title 

And somehow, there is a duplicate GUID for a certain WebPart :

The next step is to remove one of those webparts from the page – following on from the PowerShell shown above :

#get the webpart using #3
$wp = $wpm.WebParts[3] 

#check which one we've got
$wp | select id, title
 
#delete from the page - and update 
$wpm.DeleteWebPart($wp) 
$item.Update()

That outta do it – and your users will be able to browse to the page.

NB.  You might have to checkout the page also – and then verify which webpart you need to remove – be careful !