What it wrong with this function ?

Question

private void btnAdd_Click(object sender, EventArgs e)
     {
         {
             con.Open();
             //string Sql = "INSERT INTO Members ( MemberId, Name, Address, Mobile, Email ) VALUES " + " (@Id, @name, @address, @mobile, @email)";

             string Sql = "INSERT INTO Members ( MemberId, Name, Address, Mobile, Email ) VALUES " + " (@Id, @name, @address, @mobile, @email); SELECT MemberId FROM Members WHERE NOT EXISTS (SELECT * FROM Members WHERE MemberId = @Id AND Name = @name AND Address = @address AND Mobile = @mobile AND Email = @email)";

             using (SqlCommand cmd = new SqlCommand(Sql, con))
             {
                 cmd.CommandText = Sql;
                 cmd.Parameters.AddWithValue("@Id", tbID.Text);
                 cmd.Parameters.AddWithValue("@name", tbName.Text);
                 cmd.Parameters.AddWithValue("@address", tbAddress.Text);
                 cmd.Parameters.AddWithValue("@mobile", tbMobile.Text);
                 cmd.Parameters.AddWithValue("@email", tbEmail.Text);
                 cmd.ExecuteNonQuery();

                 //Sql = "INSERT INTO Payments ( MemberId, [Year], [Amount] ) VALUES " + "(@id, @year, @amount)";
                 Sql = "INSERT INTO Payments ( MemberId, [Year], [Amount] ) VALUES " + "(@id, @year, @amount) SELECT MemberId FROM Payments WHERE NOT EXISTS (SELECT * FROM Payments WHERE [Year] = @year AND [Amount] = @amount)";
                 cmd.Parameters.Clear();
                 cmd.CommandText = Sql;
                 cmd.Parameters.AddWithValue("@Id", tbID.Text);
                 cmd.Parameters.AddWithValue("@year", tbYear.Text);
                 cmd.Parameters.AddWithValue("@amount", tbAmount.Text);
                 cmd.ExecuteNonQuery();

                 MessageBox.Show("Data Added");
                 tbID.Clear(); tbName.Clear(); tbAddress.Clear(); tbMobile.Clear(); tbEmail.Clear(); tbYear.Clear(); tbAmount.Clear();
                 con.Close();
             }
         }
     }

I have 2 tables "Members" & "Payments" When I enter one new record it records its and then if I enter the same over and over again it duplicates it many times which it "SHOULD NOT" I should get warning that it already exists. some how I am stuck with this can anyone assist !