Even when I am already in the meeting, the MS Teams Desktop app, gives an option to the user at the top panel to join the meeting.
Join a meeting. As soon as you minimize the meeting, the meeting icon automatically gets minimised at the bottom left of your screen and when you open the Teams app again to do something else (for example write meeting notes) you will see that you will get an option to join that meeting again.
Please see the screenshot.
Solution: That join option should be greyed out or if I am already in the meeting, that option should not be visible to me.