Double registration

I would look into possible cache issues.

WP-Members uses several functions within WP core to validate the user, which would disallow a duplicate user as WP’s requirements are that a user have a unique value for both user_login and user_pass. If you use a cache plugin that caches database objects, that could lead to this type of issue.

If you’re using a cache plugin, make sure that you understand the configuration that you’re using. Ideally, you’d want to exclude registration from being cached.

If you (or your host) is using the server side cache called Varnish, then you for sure need to direct all registration and login to specific pages that you can exclude from the cache.

Ok, I asked the author of the simple cache plugin I use if the issue can come from it. Anyway, in its support forum I found a recent post talking about cache issues caused by conflicts with Wordfence (I use Wordfence too). Thanks.

P.S. (OT): have you ever considered to introduce my feature? ??

The feature you asked about (a math captcha) is already possible – at least in the login form. WP-Members’ login form incorporates WP’s native hooks for the login form and authentication. As long as the third party application loads on more than wp-login.php, it will be compatible.

The reg form also supports WP’s register_form action, which is what most plugins would use to add to the register form, so it should also be supported.

The login form is not a problem for me, I’m interested in the registration form: I want to add a custom field and replace the captcha (I casually found I can remove 100% of spam submission with a very stupid trick) ??

I will do some research using what you suggested as a starting point, thanks. ??