Which HTTP response code for "This email is already registered"?

I am not really satisfied with returning a 409 Conflict for an existing registered email - in my opinion, it's not a client error. So let's take a look at how some big tech companies are handling that case (at least how they are doing it in their WEB Site APIs).

Gmail (Google) returns a 200 OK and a JSON object containing a code which is indicating that the email is already registered.

Facebook is also returning a 200 OK but re-renders the content to a recovery page to give the user the option to recover his/her existing account.

Twitter is validating the existing email by an AJAX call To another resource. The response of the email validation resource is always a 200 OK. The response contains a JSON object containing a flag to indicate if the email is already registered or not.

Amazon is doing it the same way as Facebook. Returning a 200 OK and re-rendering the content to a notification page to inform the user that the account already exists and provide him/her possibilities to take further actions like login or password change.

So all these APIs returning always a 200 OK and presenting to the client/user either additional content to recover their account or an error message which is raised by the body content of the response.

Yes, 409 is the most appropriate response code here. Even though you are most likely returning 201 on success, you're still POSTing to a resource which is described as a collection, and POSTing a duplicate email is definitely a conflict with "the current state of the resource" as a collection. You should return a response body with a description of the problem, and hyperlinks to help resolve the problem, if possible.