How to stop opening a login page in (asp.net webforms) when user is already logged in the same browser in different tab - asp.net

Question

I have logged in as a user and it works fine , but when i try to open the login in a different tab in same browser it still goes to the login.aspx without the actual member page
Please help !
Aboutus.aspx
protected void Page_Load(object sender, EventArgs e)
{
if(Session["Username"] == null)
{
Response.Redirect("Login.aspx");
}
else
{
string Username = Session["Username"].ToString();
Label1.Text = Username;
}
}
protected void Button1_Click(object sender, EventArgs e)
{
Session.Abandon();
Response.Redirect("Login.aspx");
}
}
Login.aspx
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Login1_Authenticate(object sender, AuthenticateEventArgs e)
{
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString);
con.Open();
SqlCommand cmd = new SqlCommand("select COUNT(*)FROM [dbo].[Reg] WHERE Username='" + Login1.UserName + "' and Password=#pass");
cmd.Connection = con;
MD5CryptoServiceProvider md5Hasher = new MD5CryptoServiceProvider();
////create an array of bytes we will use to store the encrypted password
Byte[] hashedBytes;
////Create a UTF8Encoding object we will use to convert our password string to a byte array
UTF8Encoding encoder = new UTF8Encoding();
////encrypt the password and store it in the hashedBytes byte array
hashedBytes = md5Hasher.ComputeHash(encoder.GetBytes(Login1.Password));
cmd.Parameters.AddWithValue("#pass", hashedBytes);
var username = Login1.UserName;
int OBJ = Convert.ToInt32(cmd.ExecuteScalar());
if (OBJ > 0)
{
if (username == "admin")
{
Session["Username"] = Login1.UserName;
Response.Redirect("AdminPanel.aspx");
}
else
{
Session["Username"] = Login1.UserName;
Response.Redirect("About.aspx");
}
}
else
{
Label1.Text = "Invalid username or password";
this.Label1.ForeColor = Color.Red;
}
}
}
}

Answer 1

In Page_Load of Login.aspx do this:
if(Session["Username"] != null)
{
string username = Convert.ToString(Session["Username"]);
if (username == "admin")
{
Response.Redirect("AdminPanel.aspx");
}
else
{
Response.Redirect("About.aspx");
}
}

Answer 2

Your page laod method on Loginpage must be like this
protected void Page_Load(object sender, EventArgs e)
{
if(Session["Username"] == null)
{
}
else
{
Response.Redirect("index.aspx",false);
}
}
I hope this helps