In all Execution Report messages, FIX Server sends the trading system unique identifier in OrderID(37). OrderID(37) is unique per order book. The maximum length of OrderID(37) tag value is 32.
All application messages sent by FIX Client are checked for duplicate ClOrdID(11).
The uniqueness of ClOrdID(11) identifier must be maintained during FIX session. If orders with duration of more than one FIX session are used, FIX Client needs to cater for uniqueness across those. If two new orders are sent with the same ClOrdID(11), the second one is rejected.