Prevent Resubmissions for PHP Contact form - php

Question

This is the code for the form.
<?php
if ($_POST) {
if (isset($_POST["submit"])) {
$name = $_POST['name'];
$email = $_POST['email'];
$company = $_POST['company'];
$phone = $_POST['phone'];
$checkbox = '';
if (isset($_POST['checkbox'])) {
$checkbox = 'Yes';} else{
$checkbox = 'No' ;}
$message = $_POST['message'];
$from = 'Demo Contact Form';
$to = 'imvael#gmail.com, vnikolic1#cps.edu';
$subject = 'Message from Contact Demo ';
$body ="From: $name\n E-Mail: $email\n Company: $company\n Phone: $phone\n Opt In?: $checkbox\n Message:\n $message";
$headers = 'From: webmaster#bradfordsystems.com' . "\r\n" .
'Reply-To: ' .$email . "\r\n" .
'X-Mailer: PHP/' . phpversion();
// Check if name has been entered
if (!$_POST['name']) {
$errName = 'Please enter your name';
}
// Check if email has been entered and is valid
if (!$_POST['email'] || !filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) {
$errEmail = 'Please enter a valid email address';
}
// Check if name has been entered
if (!$_POST['phone']) {
$errName = 'Please enter your phone number';
}
//Check if message has been entered
if (!$_POST['message']) {
$errMessage = 'Please enter your message';
}
// then after the posting of the form data
// validation
// If there are no errors, send the email
if (!$errName && !$errEmail && !$errMessage && isset($_POST['url']) && $_POST['url'] == '') {
if (mail ($to, $subject, $body, $headers)) {
header("Location: thankyou.php"); /* Redirect browser */
exit();
} else {
$result='<div class="alert alert-danger">Sorry there was an error sending your message. Please try again later.</div>';
}
}
}
header("Location: " . $_SERVER['REQUEST_URI']);
exit();
}
This is the form itself
<div class="wrapper">
<div class="row">
<div class="col _66">
<?php echo $result; ?>
<form role="form" name="contactForm" id="contactForm" method="post" action="contact.php#contactForm">
<p>
We welcome your feedback and inquiries. Please use the form below to get in touch.
</p>
<div class="row">
<div class="col">
<input type="text" id="name" name="name" placeholder="Name" value="<?php echo htmlspecialchars($_POST['name']); ?>" required>
<?php echo "<p class='text-danger'>$errName</p>";?>
</div>
<div class="col">
<input type="email" id="email" name="email" placeholder="Company Email" value="<?php echo htmlspecialchars($_POST['email']); ?>" required>
<?php echo "<p class='text-danger'>$errEmail</p>";?>
</div>
</div>
<div class="row">
<div class="col">
<input type="text" id="company" name="company" placeholder="Company" value="<?php echo htmlspecialchars($_POST['company']); ?>" required>
</div>
<div class="col">
<input type="tel" id="phone" name="phone" <?php echo htmlspecialchars($_POST['phone']); ?> placeholder="Phone" required>
<?php echo "<p class='text-danger'>$errPhone</p>";?>
</div>
</div>
<div class="row">
<div class="col">
<input type="number" id="zipcode" name="zipcode" placeholder="Zip Code" value="<?php echo htmlspecialchars($_POST['zipcode']); ?>">
</div>
<div class="col">
<input id="checkBox" type="checkbox"> <span id="optInText">YES, I want a Free Workspace Evaluation!</span>
</div>
</div>
<div class="row">
<div class="col">
<p class="antispam">Leave this empty: <input type="text" name="url" /></p>
</div>
</div>
<div class="row">
<div class="col submit-col">
<p>Questions or Comments?</p>
<textarea id="message" name="message" placeholder="Enter your questions or comments here" style="height:200px"> <?php echo htmlspecialchars($_POST['message']); ?></textarea>
<?php echo "<p class='text-danger'>$errMessage</p>";?>
<button class="btn btn-dark hvr-underline-from-left" name="submit" type="submit" value="Send">Submit Request</button>
</div>
</div>
</form>
</div>
</div>
</section>
I am writing a contact form script from scratch The code is working correctly, but I am unable to prevent resubmissions if they user refreshed the page or press the submit buton more then once.
Also is it a bad practice to use self submitting contact forms?