FIX Protocol Tag 43: PossDupFlag

Y when the message may be a duplicate of one sent earlier with the same sequence number, for example when it is retransmitted after a Resend Request.

Tag number: 43
Data type: BOOLEAN
FIX versions: FIX 4.0 to 5.0 SP2 and FIXT 1.1
Location: Standard header

Practical notes on PossDupFlag

PossDupFlag = Y marks a message the session layer is sending again with its original MsgSeqNum, usually in answer to a ResendRequest.

A replayed message must also carry OrigSendingTime (122) with the SendingTime of the first transmission, and a new SendingTime (52).
The receiver checks the sequence number: if it already processed that number it ignores the message, otherwise the message fills a gap and is processed.
SequenceReset-GapFill messages sent during a resend also carry PossDupFlag = Y.

Common mistakes

Leaving out OrigSendingTime (122). Many engines reject the message with 373=1.
An OrigSendingTime later than SendingTime gets 373=10 and a Logout.
Confusing it with PossResend (97), which needs a business-level duplicate check.

Tag 43 in each FIX version

Values and message usage as defined in a specific version.

Data type

BOOLEAN: Y for yes or N for no.

Valid values

2 enumerated values across all FIX versions.

Value	Meaning
N	No
Y	Yes

Example

43=N

Illustrative value. In a message, fields are separated by the SOH character (0x01), often shown as | or ^ in logs.

Decode a message with this tag

Message types that use tag 43

PossDupFlag belongs to the standard message header, which every FIX message carries, so it is not listed against individual message types.

Field and message data comes from the QuickFIX data dictionaries for FIX 4.0 to 5.0 SP2 and FIXT 1.1, used under the QuickFIX Software License. This product includes software developed by quickfixengine.org (http://www.quickfixengine.org/). Enumerated value names are converted to plain text for reading.