Yes, a user can be logged in to the app on multiple devices or in multiple browser windows at the same time. One thing to keep in mind however is that all sessions they have open will count down the session timeout timer and if their session times out in one device or tab, it will timeout their entire session, but will not appear as having been timed out in their active window. To prevent any data from being lost, it is generally recommended to work in a single tab or window when working on things that need to be saved, and to save often.