WP User Manager - Stop registrations when the first and last name is the same

wpum_block_first_last_name_same.php

<?php

add_filter( 'submit_wpum_form_validate_fields', function ( $check, $fields, $values, $form_name ) {
	if ( ! isset( $values['register']['user_firstname'] ) || empty( $values['register']['user_firstname'] ) ) {
		return $check;
	}

	if ( ! isset( $values['register']['user_lastname'] ) || empty( $values['register']['user_lastname'] ) ) {
		return $check;
	}


	if ( $values['register']['user_firstname'] === $values['register']['user_lastname'] ) {
		return new WP_Error( 'validation-error', __( 'First and last name cannot be the same', 'wp-user-manager' ) );
	}

	return $check;
}, 100, 4 );

Save this file to your /wp-content/mu-plugins/ directory (you might need to create the mu-plugins directory).