Summary: It seems FixData fields are not supported.
Behavior: I'm seeing a message with header tags 212 (FixDataLen) and 213 (FixData) being rejected due to 'Duplicate Field: 34'. This exception appears to happen within message::decode when it sees tag 34 (MsgSeqNum) first in the header and then in value of tag 213.
Impact: This affects connectivity to CME's Drop Copy 4.0 environment where ExecutionReports etc can be embedded in 35=n (XmlNonFix) messages. It's not possible to certify connectivity without this feature.
All environments (e.g. linux, windows)