Multiple references to same entity don't get exported

Problem/Motivation

Given a multivalue entityreference field (eg. to a taxonomy term) and multiple references to the same entity, only the first occurence is exported. Because of that, the imported content does not match the original.

Steps to reproduce

1. Create a multivalue entityreference field (eg. to a taxonomy term) on a content type.
2. Create two taxonomy terms: TermA and TermB.
3. Create a node which references those terms, at least one of them multiple times in the same field (eg. TermA, TermB, TermA).
4. Export the node.
5. Import the ZIP to another site.

Expected result: the imported node references TermA, then TermB, then TermA again.

Actual result: as the ZIP does not contain the third field value, the imported node only references TermA then TermB: the second occurence of TermA is missing.

Proposed resolution

Have all the field values exported, not only the referenced entities.

Remaining tasks

Fix, review, commit, release.

User interface changes

None.

API changes

Probably none.

Data model changes

Some (hopefully minor) data model changes, most likely.