Reasons for creating this PR

This is a forward-port of PR #1673, made for Skosmos 2, to Skosmos 3. The only additional change is a minor adjustment to the PHPUnit test because getConceptInfo works a bit differently in Skosmos 3.

Link to relevant issue(s), if any

• Closes Calling removeDuplicatePropertyValues() on some elementary datatypes crashes Skosmos #1635

Description of the changes in this PR

See #1673

Known problems or uncertainties in this PR

Nope.

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)