Simplified form of the circuit is shown below.
The simplified circuit is balanced Wheatstone's bridge, so the resistance in arm DC remains unaffected.
Resistance of upper arms RU=R+R=2R
Resistance of lower arms RL=R+R=2R
Net resistance of the circuit, R′=RU+RLRU×RL =2R+2R2R×2R=4R4R2=R