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

Question

I'm creating a RESTful API for creating users that enforces unique email addresses:

Successful POST /users: HTTP 201 Created

If I POST the same email address again, what should the response code be? Is 409 Conflict the appropriate response code?

Answer 1

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.