IBM Worklight - JSONStore logic to refresh data from the server and be able to work offline

IBM Worklight - JSONStore logic to refresh data from the server and be
able to work offline

currently the JSONStore API provides a load() method that says in the
documentation "This function always stores whatever it gets back from the
adapter. If the data exists, it is duplicated in the collection". This
means that if you want to avoid duplicates by calling load() on an already
populated collection, you need to empty or drop the collection before. But
if you want to be able to keep the elements you already have in the
collection in case there is no more connectivity and your application goes
for offline mode, you also need to keep track of these existing elements.
Since the API doesn't provide a "overwrite" option that would replace the
existing elements in case the call to the adapter succeeds, I'm wondering
what kind of logic should be put in place in order to manage both offline
availability of data and capability to refresh at any time? It is not that
obvious to manage all the failure cases by nesting the JS code due to the
promises...
Thanks for your advices!