Prevent Multiple Login at the same time

Try this:
protected void Login1_LoggingIn(object sender, LoginCancelEventArgs e)
{
e.Cancel = Membership.GetUser(Login1.UserName).IsOnline;
}

or try this code

void Login1_LoggingIn(object sender, LoginCancelEventArgs e)
{
MembershipUser u = Membership.GetUser(Login1.UserName); Response.Write(u.IsOnline);
if (u.IsOnline)
{
Login1.FailureText = "A user with this username is already logged in."; e.Cancel = true;
}
else
{
Login1.FailureText = String.Empty;
e.Cancel = false;
}
}