With “Add what's new”, which is the default, nothing is duplicated. Folders, tasks and sessions that came over before are recognized and skipped, so the second import adds only what's new since the first, and Freelime tells you how many sessions that is before you click Import. Sessions already in Freelime stay as they are, even if you've changed them since, in either app. “Replace all Freelime data” works differently: it saves a backup first, deletes the folders, tasks and sessions in Freelime and imports Klokki's data again.