It marks "the elephant and bear" as incorrect (rather than "the elephant and the bear), but this is what I'd naturally say as a native English speaker. It caught me out.
Yes, you're right. In English we tend to drop that second "the" without loss of meaning. We can't do that in Welsh becuase if we dropped the "the": Dw i'n hoffi'r eliffant ac arth then it would mean "I like the elephant and a bear". You should report it.
We may never know why Duo likes that particular elephant and that particular bear, but in general:
- yr eliffant a'r arth - the elephant and the bear
- eliffantod ac eirth - elephants and bears
- yr eliffantod a'r eirth - the elephants and the bears