In songwriting there are basically three types of writers:
1) Those who write with an instrument. These writers usually write music and lyrics (like Brian or Scotty on the show).
2) Top liners. This is someone who writes melody and lyric, but generally not the music or the track (like Sonyae and Amber on the show). There are many successful top liners out there. Our head judge, Kara is a shining example of this.
3) There is a third category of writer that we do not have on the show: track writers. These are people who only write the tracks -- or the bed of music. They write beats only or beats and chord changes, that a top liner, or any writer, can write melody and lyrics on top of. Sometimes they will just deliver a track and leave, other times they will stay and have input into the lyrics and help shape the song further.
Also in Episode 1, we saw something interesting happen. One of the writers felt the pressure when writing the first hook about the city of LA, and he accidentally 'borrowed" his entire melody from Elton John's "Candle In the Wind." This certainly is not a good idea for several reasons. In the setting of this show, I imagine it would make the other writers worry you have no originality and make them reluctant to write with you. Secondly, you cannot just accidently lift someone else's song under any circumstance, especially as a pro!
That being said, I have been in countless sessions where we ask each other, "Did that line sound familiar?" Our job is to write stuff that is so hooky, it sounds like you have heard it before -- and, well, sometimes you have! We rely on each other to keep us on the straight and narrow. While I have never seen an entire song melody get lifted, I have seen snippets of other songs get incorporated. Generally it's an innocent mistake that is usually caught quickly and we all readjust and rewrite as we go, so our final product has no offending lines --most of the time. Sometimes lines get by a writer, the A&R agent, the record producer and the label -- and make it to the record! Sometimes no one raises a red flag ever, and all is well, but it can lead to lawsuits. As a songwriter, you cannot build a hook of a new song around the hook of an existing song. The original writers are then entitled to some portion, if not all of, the sales and royalties of the new song. All of this varies case by case.