For those that don't want to pay 20$ for removing duplicate mail items.
Create a new macro and add this code:
Dim t As Items
Dim i As Integer
Dim arr As Collection
Dim f As folder
Dim parent As folder
Dim target As folder
Dim miLast As MailItem
Dim mi As MailItem
Set parent = Application.GetNamespace("MAPI").PickFolder
Set target = Application.GetNamespace("MAPI").PickFolder
For Each f In parent.Folders
Set t = f.Items
t.Sort "[Subject]"
i = 1
Set miLast = t(i)
Set arr = New Collection
While i < t.count
i = i + 1
If TypeName(t(i)) = "MailItem" Then
Set mi = t(i)
If miLast.Subject = mi.Subject And miLast.Body = mi.Body And miLast.ReceivedTime = mi.ReceivedTime Then
arr.Add mi
Else
Set miLast = mi
End If
End If
Wend
For Each mi In arr
mi.Move target
Next mi
Next f
On my 1.4gHz Intel ULV for 7000 items it took approx. 5min.
The code asks for the folder to scan and then the target folder where the items should be moved to.