Generate random number without using GUID or IDENTITY in ASP.Net

Please I want to generate an Identification Number for each user. An ID that cannot be repeated with a maximum of 7 numbers. I also don't want to use GUID or IDENTITY.

Please help me check if it is okay to go with this?

private string GetAutoNumber()
{
     string numbers = "1234567890";
     string characters = numbers;
     int length = 7;
     string id = string.Empty;
     for (int i = 0; i < length; i++)
     {
         string character = string.Empty;
         do
         {
             int index = new Random().Next(0, characters.Length);
             character = characters.ToCharArray()[index].ToString();
         } while (id.IndexOf(character) != -1);
         id += character;
     }
    
     return id;
}

Then call The GetAutoNumber function on pageload like this

if (!IsPostBack)
{
    List<string> result = new List<string>();
    if (Session["Numbers"] != null)
    {
        result = (List<string>)Session["Numbers"];
    }
    string number = GetAutoNumber();
    if (!result.Contains(number))
    {
        result.Add(number);
        Session["Numbers"] = result;
        Labelinvoice.Text = number;
    }
}