Steps to reproduce the issue

Create two captcha fields in the same page through a form with type="captcha" (I use different "namespace" for each field)

Expected result

Two captcha in the same page.

Actual result

Error: reCAPTCHA plugin needs a site key to be set in its parameters. Please contact a site administrator.

System information (as much as possible)

Joomla 3.4.1

Additional comments

The fist time the captcha plugin is loaded, the _load function in the captcha library convert the plugin params from string to a Registry object, this is correct, but the second time the plugin is loaded, the library put the already converted params inside a new Registry object and the plugin params can't be used anymore.

File: libraries/cms/captcha/captcha.php
Line: 233

This issue can be fixed by checking if the plugin params is already a JRegistry object.

if (!($plugin->params instanceof JRegistry))
{
$params = new Registry($plugin->params);
$plugin->params = $params;
}

Please note. With the current version of Recaptcha (reCAPTCHA API version 2.0), you can have multiple recaptchas on one page, and this is the first step to achieve this feature.