Searching for a particular text in a combo box

I am creating a plugin in  which I load topic to a combo box. It works as follows: ComboBox1(CB1): Has two topics, upon selcting one of them  it loads  two other topics to ComboBox2(CB2).  the issue here is once a topic has been selected from CB1 before loading all the topics to CB2 a check has to be made so that duplicates copies topics are not loaded and no topics should be missed to CB2. Is there a method in which I can check the topics loaded to CB2? For instance, load the topics only  "if (! ui_.CB2->find(topics))" (which by the way I tried and did not work). If anybody has an idea do let me know.