What you are describing would only detect entire posts that are copied. If someone added a single word, the post would not be flagged as a duplicate. I have also noticed that some plagiarists like to copy parts of posts/content, sometimes from different sources. Also sorting as you describe is just another way of checking every post against every other post. This is how you would put everything in the correct order.

You could index parts of the text (for example the first word or the first letter of the first word) to reduce runtime, but the cost to check each post against every other post is still expensive.