create dict
for key, val in l1:
    if not dict[k]:
       dict[k] = 1
    else:
       dict[k]++

for key, val in l2:
    if not dict[k]:
       dict[k] = 1
    else:
       dict[k]++
let res = []
for key, val in dict:
    if val = 2:
        res.append(key);