I'm pretty sure all of Shakespeare's plays were derived from existing source material like history, mythology and previous plays. He was actively working in the pop culture of his time and couldn't meet deadlines coming up with things from scratch.
As for similiar ideas, I think 3 things are worth mentioning. First is the spirit of the times. Second is market forces that latch onto current successes. Third is technological innovations- if a computer animation program that allows for better rendering on scales and/or water comes out, it should be little surprise that Dreamworks and Pixar make fish movies around the same time.
As for similiar ideas, I think 3 things are worth mentioning. First is the spirit of the times. Second is market forces that latch onto current successes. Third is technological innovations- if a computer animation program that allows for better rendering on scales and/or water comes out, it should be little surprise that Dreamworks and Pixar make fish movies around the same time.