Prevent importer from creating duplicate of existing element

[Severino]

What is going wrong?

Description

When using the importer, (in my opinion) we should prevent the import from importing items that already exist.

Expected behaviour

When having an item already in the hierachy, the import should fail and all duplicates should be reported to the user to manually fix the entity tree or adjust the imported file

Actual behaviour

The importer creates a new item at that position with the same name.