Nothing happens if you make a reservation over the same time.
However, you can not enter two meetings at the same time, and you can only join one.
If you enter the second meeting, you will leave the meeting you have already joined.
Also, if the evictor is the organizer of the meeting, the meeting will end, so all participants in the meeting will be leaving.
To see if you can attend multiple meetings at the same time, see Can I use Zoom with multiple devices?