Duplicates detection

Right now, if a user has several copies of the same file (with the some content and `id`), only one file is displayed.
In case of deletion, only one copy is deleted and one of other copies is displayed next time.
Tags are stored for all copies at the same time due to usage of _content-addressing_.

Most likely, several copies of the same resource is a user's mistake. But it also might happen intentionally as back-up mechanism. Given that the app is supposed to be used in kinda distributed setup (using external syncing app like [Syncthing](https://syncthing.net/)) such a back-up should be redundant.

It seems like a good idea to detect such duplicates and present information about them to the user as counter on single item.
The [delete] button should be replaced by two then: [delete copies] and [delete all]. Maybe it would be simpler to suggest to the user to remove duplicates if such were found. Or preference may be added to the settings screen.

Start by tracing how the app groups resources with the same content and id, then inspect the UI handling for the single displayed item and its delete action. Compare the proposed duplicate counter, separate delete actions, suggestion, and settings preference before choosing a scope. Done means duplicate copies are detected and presented with an agreed deletion workflow.

Written by the indexing model from the issue text.