Will C# system.Random with 14 characters always be guaranteed to be unique string (hash)

I have a utility function in which I want to have as part of my URL querystring . I need to be sure that it is always unique

public static string RandomString(int length)
{
   const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ=abcdefghijklmnopqrstuvwxyz0123456789";
   var random = new Random();
   return new string(Enumerable.Repeat(chars, length)
     .Select(s => s[random.Next(s.Length)]).ToArray());
}

If say I pass in 14 for the length , This generates hash like

 YS5bwVTjwEBhFp
 sNi6EfU5rUxI2Z
 sQKhqhklw22vb2

If i'm using talking about 20,000 uses a year of this, Is it safe to say that it should always be unique?

Again

http://:mywebsite.com?id=sQKhqhklw22vb2 is how i would use it