Is the script really original? *SPOILERS*

Saw "Philomena (2013)" http://www.imdb.com/title/tt2431286 the other day, and midway through the movie it remined me of Memories in March.
One of the major plot items (i.e. the mother finding about the homosexual nature of her deceased son) is exactly same.
Although Philomena has many more twists and turns in its story, it came to my mind that whether the oscar winning 2013 movie is based on Memories in March?
But by the time credits rolled for Philomena, it said based on true events narrated from the book "The Lost Child of Philomena Lee" by journalist Martin Sixsmith.
Now the interesting part is the book was written in 2009. Whether Memories in March was also based on the same book without giving out any credits? Or is it a sheer coincidence that the plots match so much?