Python program to get all non unique elements in an array
One thing to keep in mind is your variable checked_list, which serves almost no purpose in the function, as you could do without the else block.  

The simplest thing I can think of would be just to shorten the function down to one list comprehension, instead of appending to two lists.

Revised code:

def checkio(original_list: list) -&gt; list:
    return [x for x in original_list if original_list.count(x) &gt; 1]


When running this function, these are the outputs:

checkio([1, 2, 3, 1, 3])  # Returns [1, 3, 1, 3]
checkio([1, 2, 3, 4, 5])  # Returns []
checkio([5, 5, 5, 5, 5])  # Returns [5,5,5,5,5]
checkio([10, 9, 10, 10, 9, 8])  # Returns [10, 9, 10, 10, 9]