<form Name ="form1" Method ="POST" ACTION = "userlogin.php" id="form1">
    <div id="main_body" class="full-width">
        <label>Username:</label>
        <input type = "text"
               id = "usernameLogin"
               name="pat_username">
        <label>Password:</label>
        <input type = "password"
               id = "passwordLogin"
               name="pat_password">
        <input type="submit" onclick="click_button_login()" value="Login" name="submit" id="submit"/>
    </div>
</form>

<?php
if(isset($_POST["submit"])){
    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "dbname";

    // Create connection
    $conn = new mysqli($servername, $username, $password, $dbname);

    //Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }


    $newUsername = mysqli_real_escape_string($conn, $_POST['pat_username']);
    $newPassword = mysqli_real_escape_string($conn, $_POST['pat_password']);

    $result = $conn->query("SELECT * FROM tablename WHERE patient_username ='$newUsername' AND patient_password='$newPassword'");



    if (mysqli_num_rows($result)) {
        header("Location: mainmenu.html");
    }
    else
    {
        header("Location: index.html");

    }
    $conn->close();
}
?>

if(isset($_POST["submit"]))

if ((isset($_POST["pat_username"]))&&(isset($_POST["pat_password"])))

Your form, you should working on security and eliminating auto submission, this eliminates auto submission as form contains SESSION_ID() which are unique on web browsing session.

    <Form Name ="form1" Method ="POST" ACTION = "userlogin.php" id="form1">
    <div id="main_body" class="full-width">
        <br>
        <br>
                    <label>Username:</label>
                    <input type = "text"
                           id = "usernameLogin"
                           name="pat_username"> <br>

                <br>    <label>Password:</label>
                    <input type = "password"
                           id = "passwordLogin"
                           name="pat_password"> <br><button value="<?php echo session_id() ?>" type="submit" name="login_check">Login</button>

   </div>
   </Form>

##SPRINTF EXAMPLE CODE
  //$query = sprintf('SELECT * FROM TABLE WHERE username = "%s" AND password = "%s"',mysql_real_escape_string($username),mysql_real_escape_string($password));

  #### EXAMPLE END HERE##

  <?php
  if(isset($_POST["login_check"]) && $_POST['login_check']==session_id()){
  $servername = "localhost";
  $username = "root";
  $password = "";
  $dbname = "dbname";

  // Create connection
  $conn = new mysqli($servername, $username, $password, $dbname);

  //Check connection
  if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
  }


$newUsername = mysqli_real_escape_string($conn, $_POST['pat_username']);
$newPassword = mysqli_real_escape_string($conn, $_POST['pat_password']);

$result = $conn->query("SELECT * FROM tablename WHERE patient_username ='$newUsername' AND patient_password='$newPassword'");



if (mysqli_num_rows($result)) {
    header("Location: mainmenu.html");
}
else
{
    header("Location: index.html");

}
$conn->close();
 }
?>