# Queues

For queue receivers, any messages that have been sent to receivers, but have not been acknowledged before the failover, may be sent to other receivers immediately after the failover.

A receiver trying to acknowledge a message after a failover may receive the `javax.jms.IllegalStateException`. This exception signifies that the attempted acknowledgment is for a message that has already been sent to another queue receiver. This exception only occurs in this scenario, or when the session or connection have been closed. This exception cannot occur if there is only one receiver at the time of a failover, but it may occur for exclusive queues if more than one receiver was started for that queue.

When a queue receiver catches a `javax.jms.IllegalStateException`, the best course of action is to call the `Session.recover()` method. Your application program should also be prepared to handle redelivery of messages in this situation. All queue messages that can be redelivered to another queue receiver after a failover always have the header field `JMSRedelivered` set to true; application programs must check this header to avoid duplicate processing of the same message in the case of redelivery.

Note: Acknowledged messages are never redelivered (in compliance with the JMS specification). The case described above occurs when the application cannot acknowledge a message because of a failover.
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