Hi,
Thanks for sharing website link. Your developer saying right if we have 1 website on 2 urls like
www.example.com/old and www.example.com then there will be 2 websites with same content.
So if we put /old one in maintenance mode then it is good.
Regarding siteliner duplicate content % need to debug more.Wheather it is showing duplicate inside pages on website or similar content found on other websites.
PrestaShop Tutorials Videos [How to do Tasks]
https://www.prestashop.com/forums/topic/907438-prestashop-tutorials-videos-how-to-do-tasks/