From time to time operators can notice multiple current sessions on a customer account, yet there is only one current session in the NAS. Generally this is not anything to worry about as it is usually due to a session not being closed properly. For example in the majority of cases it is related to networking glitches where the RADIUS server does not receive a packet with the information that a session is down. It does not cause any problems for data usage calculation since octets are counted up only for the actual active sessions.
Any stalled sessions are closed by the scripts on the RADIUS server after 24 hours of inactivity (there is a background script that does that once per day around midnight).
Alternatively, on the RADIUS tab you can use the "close stalled sessions" button.
If you experience an issue with these sessions where they are not closed after 24 hours you could contact firstname.lastname@example.org with an example.