Try:
Private Sub xyz()
Dim Model As Object
Dim cell As Range
Set Model = CreateObject("Scripting.Dictionary")
For Each cell In Range("Models")
If Not Model.exists(cell.Value) Then
Model.Add cell.Value, cell.Value
cboModel.AddItem cell.Value
End If
Next
End Sub
The dictionary object is similar to a single dimension array, but it has a exist method, which will return a true or false if a variable exist as a member. In the above scenaria, it loops through cells in a range and check if the value exist in the dict. object then adds if it isn't (so it will be there the next time it checks and avoids duplicates) and adds it to the combo box.