Attempts to somewhat implement
godotengine/godot-proposals#904 , godotengine/godot-proposals#9603 , godotengine/godot-proposals#10319 (comment)

Feature

Adds an indicator to tells whether a resource is shared. The button's tooltip shows the number of times a Resource was shared, along with whether there are subresources to be made unique:

Left Click on the Icon makes Resources Unique. Right Click - Recursive

Implementation

For this feature, after a lot of trial and error, I settled for reduz's approach of using a
HashMap to track how many times a Resource is referenced by a Node within a Scene.

• If a Resource is referenced more than once, you can make it unique (recursively or not).
• If there's only one instance of a Resource, it's already unique, so make_unique and make_unique_recursive is disabled.

• External Resources are not added to the HashMap — since we can't for certain indicate how many of them there are.
  Outside from that, I would separate this feature into two parts:

SceneTree Operations

In the Scene tree, you can add, delete, duplicate and paste nodes. Each time you do these operations, update_resource_count collects built-in non-empty Resources and add them to the HashMap, along with their respective Node. So basically you update the Resource counter each time you do these operations.

Inspector Operations

Each time you made a revelvant change to a property that holds a Resource (e.g Made a Resource unique, added a resource to an Array, etc), you update the Resource counter like above, inserting the Resource and its Node in the HashMap.

It uses a much "simpler" version of update_resource_count (although they could be merged but it would make using this API much more cumbersome for the Inspector and SceneTree). update_node_reference collects the Resources found withing a single property and either removes or adds to the HashMap. That way you're only dealing with a specific property is being set and not touching what's already added.

In short, The inspector part was the most tricky, as I did have to fix all the edge cases found when making unique, setting or removing Resources . As of 09/28, it's pretty clean all things considered. Using strictly update_node_reference and tweaking some pre existing code made it much cleaner than it previously was.

To-Do List

The following is a to-do list of everything that should be dealt with. There are some things left undetermined on how they should work, so I added Waiting for Input right after the task.

• Display a warning indicator for sub resources as to why they can't be duplicated (Waiting on review for input)
• Move Make Unique and Make Unique recursive funcionality to Icons (Waiting on review for input) Edit: NVM, moving functionality to icons hasnt been well supported by users in previous attempts by other PRs
• Hashmaps clears and updates itself when Nodes are Added/Removed from a scene
• Disable make_unique if there's only copy of the Resource or It's a subresource
• Update Hashmap through other means, such as using Resource Picker
• Counter not updating once you've done an operation.
• Track Subresources
• Bulk-Setting Node's Resources does not add them to hashmap.
• Duplicated Arrays full of Resources do not increase counter for some reason.
• External SubResources made unique should be included in the Hashmap, pointing to the parent's Nodes.
• Find better way of knowing whether a resource is external or not
• History mismatch when undo-redoing packed scenes if they're open on the editor. (Waiting on review for input) Seems to have fixed itself
• Empty Resources/Sub resources using the wrong icon (fixes itself after saving)
• Arrays (probly Dictionaries too) don't update counter when making unique
• Inconsistent Redo error might happen??
• Fix edge case when saving to disk/deleting resource from disk
• Dictionaries with Resources as Keys don't show an icon
• Edge Case with SubResources not showing the correct amount after adding them to a Resource already referenced by multiple nodes

Test Project

Added a test project so people interested in this feature can test it. It's a pretty dynamic feature though, so please toy around it and report any weird behavior!
indicator-test.zip