If you import bookings from a calendar that is also using ical export, (2 way connection) you will probably create duplicate bookings as they are passed back and forth between the systems.
Duplicates can be avoided by setting the import type to one of See the instructions for ical export >> "Ignore DuplicatesData Options" options. Duplicates are defined as any booking in to prevent re-exporting bookings which have been imported by the same room for the same dates, it does not consider guest name or other fields because these can be changed by the external systemiCal channel.