At least not all of the time.

I've been watching a series on the BBC called, 'HEX'. Not only is it well written, the characters are outstanding. The writers for this particular series instilled so much charisma into the two lead characters that you not only liked them, you cared about what happened to them.
By the time I'd watched five or six shows in the series, I realized the similarity between my novel, 'GRAVE RESULTS' and the series. While both stories moved in a different direction, the plots were extremely similar.
Which started me thinking. Did they purchase my books and steal the basic plot? No. Let's grow up here. I've known for years that frequently movies are released by two different production companies with the same general plot. These are simultaneous releases. So who stole from whom? The answer most likely is no one.
Much to my surprise I discovered years ago that there only about seven basic plots floating around. An example is 'ROMEO & JULIET'. How many times have you seen this same plot rehashed in movies from the time movies were invented and books were first printed up to the present day? Mostly in stories where either religion or different lifestyles offended either the intended bride's or the groom's family and everyone fought like the devil to keep the marriage from happening.
I believe that due to this phenomena, ideas float around, making it possible for two authors or screenwriters to come up with the same idea at the same time.
The stories may vary although the plots are similar. My point is, authors are constantly searching for ideas for novels, and while this might sound corny, with all of the authors in existence what are the odds that two or perhaps three will come up with the same plot at the same time. It's highly likely.
Now in an obvious case of plagiarism, my daughter sent me a novel years ago that was exactly like my second novel, 'DEATHBRINGER'. This time is was plagiarism because complete sections of my novel appeared in this horror novel written by two brothers who didn't even bother to change the wording. What dupes. Since their version went nowhere and mine was a bestseller, I decided not to sue.
What the hey, they never published another novel. In fact I felt sorry for them. The road to success as as author is not riding someone else's back.
My point is: unless someone's stupid enough to copy every work, it's hard to tell if they plagiarized your plot for either a novel or a screenplay. Consider what I've said about the seven basic plots and similar ideas floating through many minds at the same time.