Restrict user to apply for same post in ASP.Net

  protected void btnJobApply_Click(object sender, EventArgs e)
    {
        if (Session["ap_id"] == null)
            Response.Redirect("applicantsLogin.aspx");


        if (Page.IsValid)
        {
            //Alert user that he already apply for this job
            string applicantJob = "";

            con.Open();

            String queryA = "SELECT j_id FROM tbl_vacanciesapplied WHERE j_id = '" + applicantJob + "' ";

            SqlCommand cmdA = new SqlCommand(queryA, con);
            SqlDataReader drA = cmdA.ExecuteReader();

            if (drA.Read())
            {

                string message = " You already apply for this job.";
                string url = "jobDescription.aspx";
                string script = "window.onload = function(){ if(alert('";
                script += message;
                script += "'));";
                script += "window.location = '";
                script += url;
                script += "'; }";
                ClientScript.RegisterStartupScript(this.GetType(), "Redirect", script, true);

               
            }
            else
            {

                int jobID = Convert.ToInt32(Request.QueryString["j_id"]);
                int applicantID = Convert.ToInt32(Session["ap_id"]);


                try
                {
                    
                    //declare store procedure
                    string query = "applyvacancy";

                    SqlCommand cmdVacancy = new SqlCommand(query, con);
                    cmdVacancy.CommandType = CommandType.StoredProcedure;


                    cmdVacancy.Parameters.Add("@v_statusupdate", SqlDbType.NVarChar).Value = "Unprocess";
                    cmdVacancy.Parameters.Add("@j_id", SqlDbType.Int).Value = jobID;
                    cmdVacancy.Parameters.Add("@ap_id", SqlDbType.Int).Value = applicantID;
                    cmdVacancy.Parameters.Add("@v_dateapply", SqlDbType.DateTime).Value = now;


                    cmdVacancy.ExecuteNonQuery();
                    Response.Redirect("submitJob.aspx");

                    con.Close();
                }

                catch (Exception ex)
                {
                    lblMsg.Text = ex.Message; //"Error while saving the record.";  
                }
                
            }
        }
    }

Hi, regarding to the coding, how to write condition that user cannot apply the same job twice or more within the same time frame.This is because he/she already applied the job before. Do I need to check jobId = applicantId. I already done but its not working or maybe I'm missed out few step. Kindly advise for me. Currently I am developing a job application system like "Jobstreet". Thank you