Page_Load 한번만 실행

AutoEventWireup=”true” 시
When the page posts back, the Page_Load method is called. Then, once the server actually processes the page and sends you a new one based on changes, the Page_Load is called again, actually the first time on the new page sent to you.

So if you are pulling data in the Page_Load event or setting some values, enclose it in the following block:

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        if (Session["something"] == null)
        {
            Session["something"] = "1";
        }
        else
        {
            Session["something"] = null;

            //your page load code here 

        }               
    }
}

참조  : http://stackoverflow.com/questions/1094954/asp-net-page-load-being-called-multiple-times