I'll never forget when I came up with the title for Whispers in the Dark. I was walking through Perse and it came to me in a flash of inspiration. I got so excited. I thought it was brilliant! perfect! nothing could be better! It wasn't until after Whispers in the Dark got published that I saw this title has been used hundreds of times in books, films, you name it. Suddenly my oh-so-original title was not so original!
This happens all the time in fiction. Chances are someone has already come up with your title, your plot, your characters....yep, all been done before! Hell, I googled my current book's title and managed to find an unpublished peice of work called Deep Embrace that was also about a mermaid falling in love with a man. Crap!
I'm not worried though. There are no original stories in fiction. The trick is to take old concepts and take them in new, exciting directions.