Remove duplicates out of combobox

Count the number of times you've already encountered the value in your progress up the column ... if count = 0 , on rows already processed then add the current rows value (NOTE: the counting is be performed by excels "CountIf" worksheet function).

For i = lastrow To 1 Step -1
  If Application.WorksheetFunction.CountIf(.Range("A" & i + 1 & ":A" & lastrow), .Range("A" & i).Value) = 0 Then
    Me.Ritnummer.AddItem .Range("A" & i)
    Me.Produkt.AddItem .Range("F" & i)
 End If
Next i

Open the sample workbook, click the show userform, I usually populate the combobox with the Userform_initialize code.
Anyway it looks like this, you also need to clear the ComboBox each time you populate it or it will just keep getting bigger.

Private Sub UserForm_Initialize()
Dim c As Range, r As Range
Dim rws As Long, y As Integer

    rws = ActiveSheet.UsedRange.Columns(1).Rows.Count
    
    ComboBox1.Clear
    Set r = Range(Cells(2, 1), Cells(rws, 1))
    For Each c In r.Cells
        y = Application.WorksheetFunction.CountIf(Range(Cells(1, 1), Cells(c.Row, 1)), c)
        If y = 1 Then ComboBox1.AddItem c
    Next c
    
ComboBox1.SetFocus
Application.SendKeys "^{F4}"

End Sub
Private Sub CommandButton1_Click()
Unload Me
End Sub