1.

def find_dups(int_list):
    seen = set()
    repeats = set()
    
    for num in int_list:
        if num in seen:
            # we've seen this before, so it's a
            # repeat and we want to return it eventually
            repeats.add(num)
        else:
            # this is a new number so add this to the 
						# list of numbers we've seen
            seen.add(num)
    
    return repeats