We are still importing the statements manually and checking the data range each time during export. From what I understand about the online sync option, it should automatically set the last sync date and select the sync range accordingly. However, it likely does not account for statements that have already been imported manually.

Thanks for the reply. At least with my bank and integration provider transactions are getting duplicated.
An automatic deduplication on import (be it online sync or manual import) would be a helpful feature for sure.

Have you tried using the data cleaning module for this?
You could create a new deduplication rule there, set it to run automatically, and define rules for a 100% match to ensure accurate results.

We're on Odoo Entrprise, hosted on Odoo.com. Thus we cannot install modules afaik. But thanks for the suggestion.

The Data Cleaning module is a standard feature in Odoo and should be available in the Community version. To access it, simply activate Developer Mode.