CISCO_R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
CISCO_R1(config)#int Ethernet0/0
CISCO_R1(config-if)#duplex auto
                           ^
% Invalid input detected at '^' marker.

CISCO_R1(config-if)#do duplex auto
duplex auto
 ^
% Invalid input detected at '^' marker.

CISCO_R1(config-if)#duplex ?
  full  Force full duplex operation
  half  Force half-duplex operation

CISCO_R1(config-if)#duplex