check textbox for unique value

debug your javascript... one more problem is that, your select query return set of rows then how you compare "dbdata == Email_Id"?
if your javascript open database connection then write code like
var db = database.open("kc-pc\sqlexpress.CWCLondon");
var Email_Id = document.getElementById("MainContent_TabContainer1_TabPanel2_txt_email");
var dbdata = db.Query("SELECT email_address FROM call_details where email_address=" + Email_Id );

if (dbdata >= 1)
{
alert("email id exsist");
}