Translation:I think that Gareth is the tallest and that Dewi Lingo is the shortest.
I got this right except I omitted the second 'that' in the English which should be accepted. However, it was marked as wrong. The first 'that' in English is sufficient.
We actually deleted this sentence as being one that was needlessly complicated.
Sometimes they linger on in the database. As you suggest the second 'that' is not really necessary in English although it is in Welsh.
We don't want to put this sentence back into the course and so we won't be able to edit it to remove the requirement for the second 'that'.
Hopefully it will disappear soon and you'll be able to do the other sentences teaching the superlative.