functions.php

/**
 * Add the Confirm Email Address field to the Billing chekcout form
 */
add_filter( 'woocommerce_checkout_fields' , 'wc_custom_add_email_confirm_field_checkout' );
function wc_custom_add_email_confirm_field_checkout( $fields ) {
	$fields['billing']['billing_email_confirm'] = array(
		'label'    => __( 'Confirm Email Address', 'domain' ),
		'required' => true,
		'class'    => array( 'form-row-wide' ),
		'priority' => 120,
	);

	return $fields;
}

/**
 * Validate the Confirm Email Address field making sure that it matches the billing email.
 */
add_action( 'woocommerce_after_checkout_validation', 'wc_custom_check_matching_email_address', 10, 2 );
function wc_custom_check_matching_email_address( $data, $errors ) {
	$billing_email   = sanitize_email( $data['billing_email'] );
	$confirm_billing = sanitize_email( $data['billing_email_confirm'] );

	if ( ! empty( $confirm_billing ) && $billing_email !== $confirm_billing ) {
		$errors->add( 'billing', __( 'Your email address does not match.', 'domain' ) );
	}
}