Spring JMS / Integration thread routing - spring

Question

I have a Spring JMS-to-database microservice using Spring Integration to route incoming messages through a sequence of filter, transform, and service activator endpoints (the last persisting the filtered and transformed messages to an Oracle database). Messages have a primary key used in the database. We have a 5 concurrent consumer set up, based on MQ constraints in prod. All of this works.
Recently we found that the upstream producer can send two or more messages with the same primary key back-to-back. Since two separate threads are given the two messages, even MERGE SQL fails with a SQLIntegrityConstraintViolationException (both threads attempt the ON NOT MATCHED INSERT... portion of the MERGE with the database server).
So we thought to change the listener to guarantee that a specific thread would get a message with any given primary key, using a Hash of the key moduloed by the concurrent consumer count. We need to ACK the message once it is saved to the database, not before, so I want to preserve transaction semantics. But I have been unable to get this working with either a Router, or a LoadBalancer on the channel coming from our DefaultMessageListenerContainer. I've also looked at sub-classing DefaultMessageListenerContainer, but don't see the appropriate point to select a given thread.
Any recommendations?
Bean/channel XML setup, our classes renamed for brevity:
...
<bean id="listenerContainer"
class="org.springframework.jms.listener.DefaultMessageListenerContainer"
p:connectionFactory-ref="consumerConnectionFactory"
p:concurrentConsumers="5"
p:maxConcurrentConsumers="5"
p:destinationName="IN_QUEUE"
p:idleConsumerLimit="5"
p:sessionTransacted="true"
p:errorHandler-ref="errorHandler"
p:autoStartup="true"
p:recoveryInterval="60000"
p:receiveTimeout="5000"/>
...
`<int-jms:message-driven-channel-adapter id="xxx-message-in"
container="listenerContainer"
error-channel="errorChannel"/>
<int:transformer id="unmarshaller"
method="unmarshalPayload"
input-channel="message-in" output-channel="filterIt">
<bean class="com.db.xyz.endpoint.UnmarshallerEndpoint"/>
</int:transformer>
<int:filter id="identifyIt" input-channel="filterIt"
output-channel="transformItC" method="isItOurs">
<bean class="com.db.xyz.service.endpoint.ItFilterEndpoint"/>
</int:filter>
<int:transformer id="transformIt"
input-channel="transformItC"
method="transform" output-channel="persistItC">
<bean class="com.db.xzy.service.endpoint.ItTransformEndpoint"/>
</int:transformer>
<int:service-activator id="persistIt" method="publish"
input-channel="persistItC">
<bean class="com.db.xyz.ilr.service.endpoint.PersistEndpoint"/>
</int:service-activator>
Along with error channel definitions, etc. The consumerConnectionFactory is just a class that selects a Solace JMS, MQ Series, or Active MQ factory based on properties.