We can use an optional merger function also in case of same key collision. For example, If two or more persons have the same getLast() value, we can specify how to merge the values. If we not do this, we could get IllegalStateException. Here is the example to achieve this...
Map<String, Person> map =
roster
.stream()
.collect(
Collectors.toMap(p -> p.getLast(),
p -> p,
(person1, person2) -> person1+";"+person2)
);