Deleting Duplicates of Second Instance

Sourabhg98 is right, it's hard to tell exactly what you're after. The procedure below works for me, based upon what I think you want. If the number in C matches any number in A or B, the C value will be deleted. It isn't clear from your post if the duplicate occurrences will always fall in the same row, so my procedure sweeps through all rows for each number in column C. This is the less efficient path, but it should still cover everything.

Sub ClearDupesInC()
Dim LastC As Long
Dim LastB As Long
Dim i As Long
Dim j As Long


Application.ScreenUpdating = False
LastC = ActiveSheet.Cells(Rows.Count, 3).End(xlUp).Row
LastB = ActiveSheet.UsedRange.Rows.Count

For i = LastC To 2 Step -1       'if you have no header, go from LastC to 1
    If Cells(i, 3).Value <> "" Then
    For j = LastB To 2 Step -1   'if you have no header, go from LastB to 1
        If (Right(Cells(i, 3).Value, 8) = Right(Cells(j, 2).Value, 8) And InStr(2, Left(Cells(j, 2).Value, 4), Left(Cells(i, 3).Value, 3))) _
        Or (Right(Cells(i, 3).Value, 8) = Right(Cells(j, 1).Value, 8) And InStr(2, Left(Cells(j, 1).Value, 4), Left(Cells(i, 3).Value, 3))) Then
            Cells(i, 3).ClearContents
        End If
    Next j
    End If
Next i
Application.ScreenUpdating = True
End Sub