org-icalendar-store-UID

org-icalendar-store-UID is a variable defined in `ox-icalendar.el'.
Its value is

nil

Documentation:
Non-nil means store any created UIDs in properties.

The iCalendar standard requires that all entries have a unique identifier.
Org will create these identifiers as needed. When this variable is non-nil,
the created UIDs will be stored in the ID property of the entry. Then the
next time this entry is exported, it will be exported with the same UID,
superseding the previous form of it. This is essential for
synchronization services.

This variable is not turned on by default because we want to avoid creating
a property drawer in every entry if people are only playing with this feature,
or if they are only using it locally.

You can customize this variable.