This is a very generic problem and you will have to employ multiple approaches to get any respectable results. In fact what you are talking about is the ultimate goal of NLP. I suggest you break down the problem into pieces and address each piece one by one.
First piece of the puzzle is to understand if two sentences are talking about the same/similar entities. This can be done by identifying subjects, objects, verbs, location references, instrumentative references, dative references etc. in different sentences. These references then can be compared to each other. One way that comes to my mind is to look at the wordnet distance. You will have to build your vocabulary over a period of time.
The second piece of the puzzle is to then address the ethos of the sentence. You will have to employ machine learning approach here along with linguistics.
As I said, it is a VERY generic problem and thus quite difficult to solve in one go. If I were you I would address problem in following manner:
Step 1. Start by restricting my solution to one domain. This will help me build better ontology/vocabulary, train my models better.
Step 2: Resolve entity proximity and try and understand which sentences are talking about similar subjects or are pointing to similar objects etc. This step is more of a linguistic problem
Step 3: With the help of machine learning try and find sentences which have similar ethos and tonality.
Step 4: Move to next domain and repeat the steps.
Hope this helps.