What am I supposed to do?

This is my first time on your website and I'm not sure what I am supposed to do. After clicking on the Home island, I picked the very first mission listed "non-unique-elements". As I entered the code editor, I watched a very quick video and am now lost at what I'm supposed to do next. The video was not good at explaining what to do.

I've been programming Visual Basic as a full time job since 1999. I did some Python in Codecademy 2 years ago. I was expecting that the first lessons would be very basic, as in "Hello World", so I could get accustomed to the editor. I feel like I'm in an advanced lesson. It says the skill level is "elementary". Where is kindergarten?

My son is here with me. He's mostly completed the "Hello World" beginner programming book and he's just as clueless on your site. Please tell me I took a wrong turn and that I missed the rookie lessons.

Ben Pittsburgh, PA

(I never touched the code below.)

My Code:

#Your optional code here
#You can import some modules or create additional functions


def checkio(data):
    #Your code here
    #It's main function. Don't remove this function
    #It's used for auto-testing and must return a result for check.  

    #replace this for solution
    return data

#Some hints
#You can use list.count(element) method for counting.
#Create new list with non-unique elements
#Loop over original list


if __name__ == "__main__":
    #These "asserts" using only for self-checking and not necessary for auto-testing
    assert isinstance(checkio([1]), list), "The result must be a list"
    assert checkio([1, 2, 3, 1, 3]) == [1, 3, 1, 3], "1st example"
    assert checkio([1, 2, 3, 4, 5]) == [], "2nd example"
    assert checkio([5, 5, 5, 5, 5]) == [5, 5, 5, 5, 5], "3rd example"
    assert checkio([10, 9, 10, 10, 9, 8]) == [10, 9, 10, 10, 9], "4th example"