Unique validation triggered when revisiting the same anchor.

Steps:

1. Create e.g. folder.
2. In a richtext field add some text.
3. Select text and start adding anchor.
4. Enter valid anchor name.
5. Hit Save.
6. Edit this anchor again.
7. Do not modify name.
8. Hit Save.

Actual result: Unique validation fails (https://recordit.co/dnYDTbQqUh).
Expected result: The same name can be saved without errors.