If the artwork is same which is created by a multiple person for example a Boat is designed by a person and another person also designed another boat and also another person designed another boat. Then there is no problem as all the designed will not be same their must have a difference. But if any one copied from another person then that will be questionable.
Yeah that's what I was also explaining above that you can use same things in your artwork but there's no possibility that the two artwork would be identical unless you try to copy someone's else artwork to present your entry.This is just a competition and try to be creative and submit whatever you have in mind and no need to be high profile artist to participate in these contest.
There could be "similar" stuff, like the same exact image, painted by two different people for example, we have that with fruit bowls all over the world, people draw that to learn about light and many other stuff basically to cut it short. That means there are probably a million fruit bowls, doesn't mean that people copied each other, they just wanted to paint the same thing, but each has different style and talent level, if I draw it, or paint it, it will certainly be the ugliest thing ever, but if some great painter did, it would look like so real that you want to take a bite. So, if it's like that, I think it's fine, but if it's literally same, then it's a no no.