title: ListBox Item increase when postback
body: <p>I made a web part use ASP.NET. There are a listbox and two buttons on it. When you click the Button A, it would add a new item into the listbox.When clicking the Button B, it would postback the list items data. But when you click the button B, the number of listbox number becomes twice. Does anyone meet this problem before?</p>
  
  <p>Best Regards,</p>