change code of combobox to not duplicate values

Try

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

    If Target.Count > 1 Then Exit Sub
    If Target.Address <> "$C$1" Then Exit Sub

    rws = ActiveSheet.UsedRange.Columns(1).Rows.Count

    ComboBox1.List = Filter(Evaluate("transpose(if(countif(offset(a2:a" & rws & ",0,0,row(1:" & _
        rws - 1 & ")),a2:a" & rws & ")=1,a2:a" & rws & ",char(2)))"), Chr(2), 0)
End Sub