Preventing a duplicate entry

[Domino2]

I have a search textbox that progressively fills a listbox with company names which are LIKE the search text being entered. During this time, a command button becomes active allowing the user to add a new company. However I want the button disabled if the listbox narrows down to one item, and matches what the user has entered. In other words when the listcount moves to having one item in it, is there a covert way to examine a match between the listitem and the text in the searchbox? Thanks