Great topic
There's nothing you can do that can't be done, No song you can sing that can't be sung. (All you need is Love, The Beatles)
We covered a bit of this very thing here - http://forums.songstuff.com/topic/10862-is-it-better-to-be-self-taught-or-to-get-lessons/?p=199733
Just last night I was watching a Documentary on Nile Rodgers. (Chic, Sister Sledge, Duran Duran, David Bowie, Madonna, Diana Ross and many more) He was the hitmaker. He'd write, arrange, and produce other artists while backing them up on rhythm guitar. He was a conscious writer. He would take elements of real life whether his or someone else's or even someone else's material and filter it. When he wrote "Good Times" for Chic. He wrote it based on the song "Happy Days Are Here Again"
http://www.youtube.com/watch?v=qhkk1tZzNjk
http://www.youtube.com/watch?v=6AB2Y-ZsEak
So we are guided by our unconscious or conscious. Often when we are guided by our unconscious what we are doing is trying to remember a song we may have heard before. If you've heard a song three times ever... It's stored somewhere in your brain but may be buried by other information that comes later. Songs never come fully formed in the brain. They evolve in time. When you become conscious that you may be touching on some pre existing material you can either throw the baby out with the bath water or you can evolve the song into something new. Songwriters, Novelists, Playwriters and Screenwriters do the same thing.