Using the ajax tab feature

I am developing a form using the ajax tabs and update panels to prevent postbacks. My main problem is this: When a user enters his or her last name into the text box and clicks the submit button all is well until I go to another tab where there is a dropdown box containing the last names is displayed, but because I am avoiding the postback, the dropdown box does not contain the newly entered last names. I have a Sub routine that loads the last names on page load, but if I add it to the button click event, the data is loaded again; therefore, displaying it twice. How do I get the data to only refresh the new last name without adding duplicates to the dropdown box? I am pretty sure that I should use an if statement, but I am not sure where to put it or how to construct.

Thanks