This was a point raised by Irma Boom (book designer). I think that within the area of Graphic Design there is definitely elements that people lend from one another, whether this is done on a conscious or sub-conscious level i don't know. Joe Gilmore (Leeds based Graphic Designer) gave a talk on his work and suggested as a Graphic Designer you are part of a community and you tend to follow trends. I have to agree with this on some levels because it is part of the way i work, but part of me always has that drive to want to produce new stuff.
There are many factors that come into this. Time, i would say is one of the biggest. If you are given something to complete in 1hr it could be massively different to something that is achieved in one week. You could see a trend in things created in a short period of time to that of a week. In this case then you are more likely to mimmick something else.
However, if you are someone that continually challenges your design process regardless of a brief or not, then i believe you can start to fill your bag of tools (your sketches, experiments) to use on future work, to potentially avoid any ripping off.