Cannot redeclare class WP_HTTP_Response

I’m attempting to generate an oauth token and getting the following error:

PHP Fatal error: Cannot redeclare class WP_HTTP_Response in /var/www/.../wp-content/plugins/rest-api/core/wp-includes/class-wp-http-response.php on line 15

Any ideas what to do?

https://ww.wp.xz.cn/plugins/json-rest-api/