Insert custom data to custom table on wordpress database

i need some help here

i wanna create some form for user to subscribe our article and store the data to database. i have tried to search it on google, but nothing worked when i tried that. i know that some plugin can do that but i dont know how to create the validation when user already subscribed and the form never show again.

how do i do to insert the data to my database?
hope you guys understand what the problem and any help will appreciate

thank you

here is my code

the form

                    <form action="<?php echo site_url() . '/insert-data.php'; ?>" method="POST" name="form-subscribe">
                        <div class="row">
                            <div class="col-12 col-lg-12 col-md-12">
                                <input type="text" name="subs_name" placeholder="Full Name*" required="">                               
                        <div class="row mt-3">
                            <div class="col-12 col-lg-12 col-md-12">
                                <input type="text" name="subs_email" placeholder="Email Address*" required="">                              
                        <div class="row mt-3">
                            <div class="col-12 col-lg-12 col-md-12">
                                <p><button type="submit" class="btn btn-primary" name="submitForm">Submit</button></p>

the action file aka insert-data.php


    //setting up the form
    function insertuser() {

      $name   = $_POST('subs_name');
      $email  = $_POST('subs_email');  

      global $wpdb; 
      $table_name = $wpdb->prefix . "subscriber";
      $wpdb->insert($table_name, array('subs_name' => $name, 'subs_email' => $email) );

if( isset($_POST('submitForm')) ) insertuser();


Here is the table of wp_subscriber
enter image description here