Message box if cell is duplicate

Try this:

Add sub below to your module and amend your sub
replacing:

MsgBox "Done"

with:

call AnyDuplicates

Sub AnyDuplicates()

Dim cel As Range, rng As Range
Set rng = Range("Z1").Resize(Range("Z" & Cells.Rows.Count).End(xlUp).Row)

For Each cel In rng
    If cel.Value <> "" And cel.Row <> WorksheetFunction.Match(cel.Value, rng, 0) Then
            MsgBox "Duplicate found": Exit Sub
    End If
Next cel

End Sub