ASX has identified a small number of instances where participants have received duplicate EIS164 (Notified Broker Broker Trade) messages. 

These messages have unique Message Sequence Numbers (MSNs) but contain details that have previously been communicated to the participant. 

ASX's investigation has determined that duplicate EIS164 messages may occur under limited recovery scenarios following certain service interruption or recovery activities. In these circumstances, a message may be retransmitted during recovery processing after it has already been delivered. 

While CHESS contains controls designed to identify and suppress duplicate messages, ASX has identified a circumstance in which the existing duplicate detection mechanism may not identify all duplicate EIS164 messages. 

To date, the issue has affected only a limited number of participants and occurrences remain infrequent. 

Interim Management Arrangements

ASX has implemented operational monitoring and response procedures to minimise participant impacts while permanent enhancements are being delivered. 

Where monitoring identifies a potential duplicate EIS164 message: 

1. ASX operational teams investigate and confirm the event. 
2. ASX contacts affected participants as soon as practicable to advise which message should be disregarded. 
3. Participants should continue processing the original valid EIS164 message and apply their standard reconciliation and exception-management procedures. 

Remediation Plan

Enhanced Duplicate Detection

ASX is enhancing CHESS duplicate detection controls to further strengthen the identification and suppression of duplicate messages before they are distributed to participants. 

Implementation of this enhancement is currently targeted for the end of September 2026. 

Ongoing Monitoring

ASX continues to actively monitor for duplicate EIS164 events and has established operational processes to identify, investigate and manage any occurrences. 

Where a duplicate EIS164 message is identified, ASX will contact affected participants and provide guidance regarding the message that should be disregarded. 

What do I need to do by when?

Participants are not required to undertake any testing, system changes or additional activities in response to this notice. 

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