The first thing to do is to backup your calendar, or Thunderbird profile as described in this FAQ topic
Then ensure that the preview of operations before synchronization is enabled: use menu File/Preview options (the option must be checked in blue), or the "Preview operations" icon on the right of the main synchronization progress bar (the button is COLORED if the preview is enabled)
This way you can check that all the operations performed during synchronization suit your wishes.
To avoid duplicates when you synchronize the first time with a third source, it is usually recommended to start with an empty calendar in this source. This way, it will be populated with synchronization and no duplicates will be created.
Also do not synchronize the same remote calendar (for instance a Google, ICS or calDAV calendar) accessed from different sources. BirdieSync will not know that you actually synchronize the same remote calendar and it could lead to duplicates or synchronization cycles. Only synchronize with a local calendar.
If you want to protect your calendar during first synchronization, you may set it as Read Only in BirdieSync Configuration.
For instance if your reference/master calendar is present in Thunderbird on your Notebook, in BirdieSync server Configuration, you may:
- Select Configuration view
- Select Thunderbird (on your Notebook) source in "Sources column"
- Select Events in "Contents" column
- Select your synchronized calendar in Calendar list, and then press the button with the 2 vertical arrows on the right above the calendar list, and select "Set event calendar permission to Read only".
This way, synchronization will not modify this calendar and all modifications performed in the other calendars synchronized with this one, will be reverted (they will only be a copy of this reference/master calendar)
I suggest you to have a look at BirdieSync Help (menu "Help/Help...") in section Configuration/Folder and section Configuration/Permission for more information.
Note that the Assistant to configure synchronization doesn't manage permissions. So if you use it, it may be necessary to manually set the permissions each time the Assistant is used.
Let me know if you still have questions.