Combobox add item not listed yet

[mqdias]

I have the following code that is suposed to add items to a combobox from a excel column:

Private Sub UserForm_Initialize()
With ComboBox1
For Row = 6 To 3000
If IsEmpty(Folha1.Cells(Row, 3)) Then
    GoTo Saida
    Else
    .AddItem Folha1.Cells(Row, 3)
End If
Saida:
Next Row
End With
End Sub

My problem is that in the selected column ("C") i have the same value more than once. So i want it to not repeat it in the droplist.
How can i do it?

Thanks!!