The issue of the guest being overwritten occurs when you click Edit on the guest name rather than removing the guest from the booking and adding a new one.
For example, if there are two bookings for a guest named 'James Malcom'. If you open one of the bookings, click Edit on the guest name and change it to 'Austin Powers', both the bookings will now be under 'Austin Powers' because the guest wasn't changed, only the guest's name was changed.
To unlink guest names:
Open the booking from the 'View Booking' page
Then find the room and guest you want to update and click on the red X
Do not update it from the 'Edit Booking' page
If you would like to add a different guest to the booking, you will need to click the 'x' icon to remove the guest first and then click 'Add guests details' to add a new guest.