SqlStorage override existing data

Hi, I’m trying to use the SqlStorage feature to provide a bookmarking functionality in my app. So user can bookmark a WordPress post on his device and find it quickly later. I used providers to handle the whole process. The problem is that a new bookmark overrides the existing one so I always have one item it the storage. Also I’d like to test whether an item is already bookmark in order not to save it again. Any help please ? Secondly, I’m fetching data from a WordPress website via a REST API. And and I’d like to perform two things : 1. How to make the difference between recently fetched data from previous ones ? 2. When displaying all posts, how to test bookmarked ones and show them differently in the list loop? Thanks for your help.