ODME re-migrates Zimbra contacts each time it processes them.
Zimbra does not store LastModificationTime for contact items. That means there is no opportunity to know whether a contact was modified since it was being processed by the product last time. To avoid situations where changes of contact items are not migrated, ODME re-migrates contacts each time it processes them.
This behavior should not cause any issues since the matching works fine between source and target and no duplicates are created during subsequent migrations.