Cryptomnesia

from Wikipedia:

Cryptomnesia, or “concealed recollection,” is the name for a theoretical phenomenon involving suppressed or ‘forgotten’ memories. It refers to cases where (apparently) a person believes that he or she is creating or inventing something new, such as a story, poem, artwork, or joke, but is actually recalling a similar or identical work which he or she has previously encountered. According to the theory of cryptomnesia, the person is not engaging in plagiarism, but is rather experiencing a memory as if it were inspiration.

I’m a believer despite the lack of scientific evidence simply because I’ve experienced it.  I’m sure many people have had a similar experience where you write something and than later go back to it and realize that it came from somewhere else.  Memories in the mind are like water in the ocean.