With this feature you can add the active period of the same treatment package.
1. Choose Setup Menu - More menu - Module
2. Choose Treatment - Activate at Previous Package Expiry
3. if it has been activated then if there is a customer who purchases the same package but the previous package is still tormented the active period of the newly purchased package will not be calculated / start if the previous package has not been expired
here I have 1 active package with an active period until April 19 2019 when I checkout with the same package, the result will be like this
the second purchase package will be active after the first package is expired or finished