I think it would be relatively easy to create a macro to identify duplicated sentences if they are exact matches. If every sentence was loaded to a dictionary, you would get an error if the key already exists. It would only take a single character to differ to render this method invalid though.
Is this what you are looking for? It should highlight any sentence that has already been used in the document (ignoring case, spaces, tabs, periods)
Code:
Sub RepeatOffenders()
'Requires a reference to Microsoft Scripting Runtime
'Highlights sentence repeats in document
Dim aSent As Range, sText As String, aDict As Scripting.Dictionary
Set aDict = New Scripting.Dictionary
For Each aSent In ActiveDocument.Range.Sentences
sText = Replace(LCase(aSent.Text), vbCr, "")
sText = Replace(sText, " ", "")
sText = Replace(sText, ",", "")
sText = Replace(sText, ".", "")
sText = Replace(sText, vbTab, "")
'Debug.Print sText
On Error GoTo Plagiariser2000
aDict.Add sText, sText
Next aSent
Exit Sub
Plagiariser2000:
aSent.HighlightColorIndex = wdBrightGreen
Resume Next
End Sub