Prevent (Avoid) duplicate record insert in same Date using C# in ASP.Net

If user enter more than one value to same date then value not entered in table.

If date change then value entered in table in asp c#

namespace ProjectBarcode
{
    public partial class WebForm4 : System.Web.UI.Page
    {
        string connStr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        SqlCommand com;
        string str = null;
 
       // SqlConnection con = new SqlConnection(@"Data Source=10.1.246.4;Initial Catalog=DB_NTPC_MEJA_INTRANET; User ID=dba_ntpc_meja_intranet; Password=alpha$890; Connection Timeout=15;Connection Lifetime=0;Min Pool Size=0;Max Pool Size=100;Pooling=true;");
       // string strConnString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        protected void Page_Load(object sender, EventArgs e)
        {
 
            namecheck();
            if (!IsPostBack)
            {
 
            }
        }
 
 
        public void namecheck()
        {
            SqlConnection con = new SqlConnection(connStr);
            con.Open();
            str = "select count(*)from trans where Workman_ID='" + TextBox1.Text + "'";
            com = new SqlCommand(str, con);
            int count = Convert.ToInt32(com.ExecuteScalar());
            con.Close();
 
            if (count >1000000)
            {
                Label1.Text = "Sorry! Details has been already submited..";
            }
            else
            {
                Label1.Text = " Details has been submited !";
                con.Open();
                str = "insert into trans(Workman_ID) values('" + TextBox1.Text + "')";
                com = new SqlCommand(str, con);
                com.ExecuteNonQuery();
                con.Close();
 
            }
        }
    }
}