Prevent Duplicate Entries

I have a workbook that I need to prevent duplicate entriesw in colum A I have worked out how to do this for data that is manually entred but need to do this for bulk data that is pasted in from another workbook
The code that I am currently using is :

Private Sub Workbook_SheetChange(ByVal Sh As Object, _
ByVal Target As Range)
If Target.Cells.Count > 1 Then Exit Sub
If Target.Column <> 1 Then Exit Sub
Dim mySh As Worksheet
Dim mySum As Integer
mySum = 0
For Each mySh In ThisWorkbook.Worksheets
mySum = mySum + Application.CountIf(mySh.Range("A:A"), Target.Value)
Next mySh
If mySum > 1 Then
MsgBox "Invoice Already Exists"
With Application
.EnableEvents = False
If MsgBox("Do you want to Enter that anyway?", _
vbYesNo) = vbNo Then
.Undo
End If
.EnableEvents = True
End With
End If
End Sub

what do I need to change or add to either prevent or highligth duplicats that are pasted in to the workbook??