Sub DuplicateSelectedWords()
    
Dim oRng As range
    If Len(Selection.range) = 0 Then
        MsgBox "Select the text first", vbCritical
        Exit Sub
    End If
    Set oRng = Selection.range
    oRng.Copy
    oRng.InsertAfter (oRng.text)

End Sub

Sub DuplicateSelectedWords()
Dim Rng As Range
With Selection
  If .Type = wdSelectionIP Then
    MsgBox "Select the text first", vbCritical
    Exit Sub
  End If
  Set Rng = ActiveDocument.Range(.Words.First.Start, .Words.Last.End)
End With
With Rng.Duplicate
  .Collapse wdCollapseEnd
  .FormattedText = Rng.FormattedText
End With
End Sub