You can check if the user is already logged in with User Status:

www.selfget.com/documentation/jbackend-joomla30/user-module-api.html

Example (REST format)
<end-point>/get/user/status

Response
{
"status": "ok",
"is_guest": <0 or 1>,
"user_id": "<user_id>",
"session_id": "<session_id>",
"session_expire": <session_expire>
}

Kind regards,
Luigi