I am really not clear about your question.But For example :
if you have a listbox before you add something in it get all entries to an array with :
ArrayList myarray=new ArrayList();
for(int i=0;i<listbox1.items.count;i++)>
{
myarray.Add(listbox1.Items[i]);
}
Then write a function to f,nd same entries.It should work with two rotation.I mean you should control all of entries with every one.If there is a same entry you should handle it for example with a return; code part.Else you can add your suffix like :
int sameindex;
listbox1.Items[sameindex]+=" +Mysuffix";
I did not try this code block.But it will work ^^