MIB: EtherLike-MIB.
The object name: dot3StatsDuplexStatus.
The current mode of operation of the MAC entity 'unknown' indicates that the current duplex mode could not be determined.
The management control of the duplex mode is accomplished through the MAU MIB. 
When the interface does not support autonegotiation, or when autonegotiation is not enabled, the duplex mode is controlled using ifMauDefaultType.
When autonegotiation is supported and enabled, duplex mode is controlled using ifMauAutoNegAdvertisedBits.
In either case, the currently operating duplex mode is reflected in both: in this object and in ifMauType.
Note that this object provides redundant information with ifMauType.
Normally, redundant objects are discouraged. 
However, in this instance, it allows the management application to determine the duplex status of an interface without having to know every possible value of ifMauType.
This was felt to be sufficiently valuable to justify the redundancy.
For the reference see: [IEEE 802.3 Std.], 30.3.1.1.32, aDuplexStatus.