Thank you for submitting a ticket with us.
When synchronization is performed for the first time, it does not check whether the records in sources are identical and simply copies data from one side to another and vice versa, and "maps" the original records to their copies in another source.
During subsequent synchronizations, if a record is modified or deleted in one source, Skyvia modifies or deletes the data, mapped to this record in another source, and vice versa.
Skyvia builds the id map when inserting data, and remembers it, and when performing next synchronization, Skyvia knows how the records correspond to each other.
So, in your case if you create an Import package and transfer the records with CSV, then create a Synchronization package and run it, it would result in duplicated records both in source and target.
We would advise going with synchronization right away. You can find more details at link
We are looking forward to hearing from you, should you have any further questions.