FIX Error: Duplicate MsgSeqNum – Explained
A duplicate MsgSeqNum occurs when the same sequence number appears twice in a session. This can indicate message retransmission, sequence number generation errors, or application bugs.
Root Cause
1) Message retransmitted with same sequence, 2) Sequence number generator produces duplicates, 3) Application restart reuses sequence numbers, 4) Sequence reset not properly handled, 5) Multiple instances sending with same sequence.
Broker/Exchange Action
Broker may reject duplicate messages or handle them based on PossDupFlag (43). Some brokers accept duplicates if PossDupFlag=Y.
Client/Your Action
1) Ensure sequence numbers are unique and incrementing, 2) Track sent sequence numbers to avoid duplicates, 3) Use PossDupFlag=Y for retransmissions, 4) Fix sequence number generation logic, 5) Prevent multiple instances from using same sequence.
Example FIX Message
How FIX Doctor Detects This
FIX Doctor tracks MsgSeqNum values and flags when the same sequence appears multiple times. Duplicates are highlighted in the error summary.
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