Compare two topic modelling sets
Considering you extracted the news using a TF-IDF approach what you have as a result is just one feature (frequency of terms). I think you would need to add more features to your corpus to be able to match two news as the same (or similar).

One new feature would be the temporal, where you would add a timestamp for the news. It will allow you check if two news (from different publishers) were published on the same period. (a week, 2 weeks, etc).

The second one could be a spatial, if you have geolocation of the news for example, you can add it to your train dataset. Something similar was done by Chi-Chun Pan. It would allow you be more confident if the news happened on the same place.