It's definitely redundant - but it's a very common phrasing in Swedish, so we need to teach it, which means we need to employ unidiomatics. And since we already use that for the default translation, it makes little sense to not accept the same construction for the other translations.
The words de and dem are both pronounced as though they were written dom, but they're usually written as de and dem. In more informal writing, it's also OK to write dom, but in the dictation exercises, the system is incapable of accepting any spelling variation at all. So if you type dom in translation exercises, it's usually an accepted answer, or should be, but in the dictation exercises it never works.
mark, standard English distinguishes between "lie" and "lay". The former is intranstive, the latter transitive. So:
1. He lays the book on the table.
2. The book is lying on the table.
But incorrect are:
3. He lies the book on the table.
4. The book is laying on the table.
Some native speakers nowadays do say 4, but it would still be marked wrong in, for example, a university-level paper.