How to aggregate messages with the identical headers in Spring Integration

public static void main(String[] args) throws InterruptedException {
    AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
    ctx.register(Main.class);
    ctx.refresh();
    DirectChannel channel1 = ctx.getBean("channel1", DirectChannel.class);

    ctx.getBean("channel2", PublishSubscribeChannel.class).subscribe(message ->
           System.out.println("Output: " + message));

    channel1.send(MessageBuilder.withPayload("p1")
        .setHeader(CORRELATION_ID, 1)
        .setHeader(SEQUENCE_SIZE,2)
        .setHeader(SEQUENCE_NUMBER,1)
        .setHeader("a", 1)
        .build());

    channel1.send(MessageBuilder.withPayload("p2")
        .setHeader(CORRELATION_ID, 1)
        .setHeader(SEQUENCE_SIZE,2)
        .setHeader(SEQUENCE_NUMBER,2)
        .setHeader("a", 2)
        .build());
}

    @Bean
    public MessageChannel channel1() {
        return MessageChannels.direct().get();
    }

    @Bean
    public MessageChannel channel2() {
        return MessageChannels.publishSubscribe().get();
    }

    @Bean
    public IntegrationFlow flow1() {
        return IntegrationFlows
                .from("channel1")
                .aggregate(a -> a
                        .releaseStrategy(new SequenceSizeReleaseStrategy())
                        .expireGroupsUponCompletion(true)
                        .sendPartialResultOnExpiry(true))
                .channel("channel2")
                .get();
    }

Output: GenericMessage [payload=[p1, p2], headers={sequenceNumber=2, a=2, correlationId=1, id=b5e51041-c967-1bb4-1601-7e468ae28527, sequenceSize=2, timestamp=1580475773518}]

Headers "a" and "sequenceNumber" were overwritten. How to aggregate messages with the identical headers?

It must be so

Output: GenericMessage [payload=[p1, p2], headers={sequenceNumber=[1,2], a=[1, 2], correlationId=1, id=b5e51041-c967-1bb4-1601-7e468ae28527, sequenceSize=2, timestamp=1580475773518}]