Session stores old value using captcha

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ASP.NET

Hi, When its a postback somewere on my page my captha session still has the old value so when the captha get updated on postback the old value is still there

So my textbox value could not be true to the captcha value

What should i do, to clear my Session and take the new value form the captcha?

Here is my code

protected void Button1_Click(object sender, EventArgs e)
      {
          var gettxtimagecode = txtimgcode.Value; // textbox value that the users enter
      check if textbox value == CaptchaText
          if (gettxtimagecode == this.Session["CaptchaImageText"].ToString())
          {

          else
          {
              lblmsg.Text = "Image text is wrong!";
          }
          this.txtimgcode.Value = "";

      }