Does the add-on modify any content or attachments?
When are attachment identified as duplicated?
Attachments will be marked as duplicated if they have the same content.
Is there are a continuously running background job to check new attachments?
Only the installation starts a background job which checks all existing attachments. The job runs asynchrone and by careful utilization of existing resources to do not slow down the system! Updates e.g. in case of new attachments are event based triggered.