cf7-prevent-multi-submit.php

// Prevent Multi Submit on WPCF7 forms
add_action( 'wp_footer', 'mycustom_wp_footer' );

function mycustom_wp_footer() {
?>

<script type="text/javascript">
var disableSubmit = false;
jQuery('input.wpcf7-submit[type="submit"]').click(function() {
    jQuery(':input[type="submit"]').attr('value',"Sending...")
    if (disableSubmit == true) {
        return false;
    }
    disableSubmit = true;
    return true;
})
  
var wpcf7Elm = document.querySelector( '.wpcf7' );
wpcf7Elm.addEventListener( 'wpcf7submit', function( event ) {
    jQuery(':input[type="submit"]').attr('value',"send")
    disableSubmit = false;
}, false );
</script>
<?php
}