"I'll see my girlfriend next year."
They are. Honestly I'm not sure why Duolingo has you put 我明年 instead of 明年我. I lived in china for two years and they almost always put the time at the begginning of the sentence. My Chinese friend told me it felt more natural that way. But I've also noticed that there are a few oddities like this in the Duolingo Chinese. It coukd be they're only teaching textbook Chinese and not natural Chinese, or it could be a geopraphical difference as in north vs south. Idk, its weird
I had the same question - my best theory is that maybe there is a difference in meaning in this case, since the timeframe is long and vague.
Without the possessive, it might imply you are going to meet a hypothetical new girlfriend next year, whereas with the possessive, it implies you are going to meet your currently existing girlfriend? I suppose this also depends on the exact meanings of 见, which I am not totally clear on.
Or, Duolingo just isn't accepting a correct input again...