def remove_list_duplicates(some_list):
    '''Returns a new list that has the duplicates of some_list removed.'''
    return list(set(some_list))


def find_duplicates(some_list):
    '''Return a set of numbers that appear at least twice in some_list.'''

    elem_set = set() # all elements in the list
    dups_set = set() # just the duplicates
    
    for elem in some_list:
        if elem in elem_set:
            dups_set.add(elem)
        else:
            elem_set.add(elem)

    return dups_set


def find_duplicates2(some_list):
    '''Alternate implementation of find_duplicates.'''
    
    dup_list = some_list.copy()
    for elem in set(some_list):
        dup_list.remove(elem)
    
    return set(dup_list)