Currently Materials and its linked resources are appended. When a material is used multiple times the resources are appended again. Inside blender Images (unique id) are used as key for many caches (including GPU textures).
This task will link images:
* Reducing the amount of needed resources.
* Declutter the image list.
* When having an image multiple times in the list they can get out of sync in blender and would also confuse users which one has local changes.
Materials will still be local and the image nodes can still be changed. settings in the texture nodes are stored in the image (due to caching) So interpolation would be grayed out.
NOTE: In stead of linking we could also deduplicate local images.