highlighting for not duplicates items

Speed

In your test, variable fn can never be  < 1 since the CountIf function will always return 1 or more (since the cell itself is at least one instance of its own value in column A).

You just need to make the changes in bold below (done in the attached file):

Sub Macro2()
Dim lr, fn As Integer
lr = Sheet1.Range("A" & Rows.Count).End(xlUp).Row

For i = 2 To lr
    fn = Application.WorksheetFunction.CountIf(Sheet1.Range("A:A"), Sheet1.Range("A" & i).Value)
    If fn = 1 Then Sheet1.Range("A" & i).Interior.ColorIndex = 3
Next i

End Sub

then all unique values will get a red fill (but others will remain as they were). You can restore the block If (should you need to do more than fill the cell) but currently it's okay on one line.

This is because ColorIndex has colours numbered 0-56 (plus automatic and none) where 3 is red. 

Alternatively, you could use the line:

If fn = 1 Then Sheet1.Range("A" & i).Interior.Color = vbRed

Hope this helps.