There are literally many thousands of people writing scripts and submitting them each year, but less than 400 films are made each year, plus TV episodes. A writer's skill is not so much in forming the basic plot, but in creating characters, a story line, and fleshing it out. It is very likely that writers will see something similar to their basic plot on TV within the next one to three years, or even a similar story line, especially if the topic is hot, or very universal. This doesn't mean their script has been plagiarized, or indicate a lack of creative talent in Hollywood, or even that someone stole their basic idea. It simply indicates the reality of the creative world.
Great minds think alike.
If you believe actual plagiarism has occurred within the film industry, you can contact a lawyer or the Writer's Guild for sound advice on how to proceed to rectify the situation. East of the Mississippi River, contact Writer's Guild of America, East, Inc. 555 West 57th. Street, New York, NY 10019-2967. Beverly Boulevard, Los Angeles, CA 90048. The Writer's Guild is a bargaining agency for members with produced screenplays , but can provide limited information to non-members.