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The oxford dictionary stated that there are around 171,476 words currently in use. Obviously we should narrow down the number of words used in a given sentence here due to grammatical constraints and forum context.
According to
lingholic, a vocabulary of 3,000 words will cover 95% of "common texts" and the average adult has a vocabulary of about 20,000 words.
If you take the 3,000 words that make up 95% of "common texts" (books, news articles, blog posts), and square this figure, there are 9 million possible word combinations. This means if you write two random words, there is a 1 in 9 million chance someone else will write those same words also writing two random words. Granted, some of the rules of English grammar dictate that certain words cannot be used after other particular words. I believe there to be possibly be about 5 million posts in this forum, if I am understanding how the "msg" number works in SMF correctly.
Some words are used more frequently than others, and some words will be used more frequently when discussing a particular topic/subject. I think it is pretty unlikely that two people will write the same thing, with the 2nd person writing 10 or more words exactly as the first person wrote without ever interacting with the 1st person's post.
There are certain common phrases that are okay, but I have not seen anyone banned for using a common phrase.