# find all elements that exist in both lists

# original lists
a = [1,2,3,4,5]
b = [2,3,10,11,12,1]

# create 3rd list
c = []

# look at all elements in a
for num in a:
    # see if they appear in b
    # to avoid duplicates, check that it's not already in c
    # add to c
    if num in b and num not in c:
        c += [num]

print(c)