Copy of localized object has wrong l18n_parent
If you copy a translated element and paste it in to the same or another site, the field
l18n_parent (or in extensions
l10n_parent) is filled with the value of the original object (before_paste.png).
So the new copy is not shown in the backend (after_paste.png). This means that it can't be deleted by an editor.
The copy exists only in the database (database_lookup.png).
In TYPO3 8 and lower you the option
mod.web_list.enableLocalizationView can be set, which prevents the user from copying localized records.
Unfortunately, this option has been removed/does not work in TYPO3 9LTS.