No, in general, sync will not create duplicates. Sync is smart enough to detect duplicates and automatically merge them, so you don't need to worry about it.
A synced transaction is considered a duplicate if all of the following conditions are met:
- Amount exactly matches a previously entered transaction.
- Description is similar to the previously entered transaction. Similarity is calculated as the percentage of words that are common to both transactions.
- Date is within 5 days of the previously entered transaction.
If you enter transactions manually in a "Pending" state. Upon syncing, they will be automatically marked as "Reconciled".