Embedding the same file twice not setting correct "delta" on future edits

Problem/Motivation

The functionality introduced by #2308451: Only one view mode possible per file per field works OK when all of the files are embedded at the same time, but there is a problem when an already-embedded file is embedded a second time on a future edit. The "delta" that is generated is not unique. This has the effect of making the 2 embeddings clone each others' overridden fields, rather than maintaining 2 separate sets of overridden fields.

It's a very niche case, which is why I have this at "minor" priority.

Proposed resolution

Write a patch to ensure that we keep track of the "delta" of existing embeddings so the newly added duplicates can have unique field values.

Remaining tasks

Write a patch.
Review patch.

User interface changes

None.

API changes

None.

Data model changes

Does affect the embedded tokens in that it prevents erroneous sharing of field values between duplicate embedded files.