from collections import Counter
boys = ["mark", "john", "mark", "fred", "paul", "john"]
girls = ["mary", "joan", "joan", "emma", "mary"]
b = Counter(boys)
g = Counter(girls)
c = b + g
#result: c = Counter({'mark': 2, 'joan': 2, 'john': 2, 'mary': 2, 'fred': 1, 'paul': 1, 'emma': 1})