Find the equivalent resistance between A and B

Symmetry about XY, X and Y are same potential make new diagram by placing X and Y at one place

${\text{R}}_{\text{eq}}\text{=}\left[\frac{\left(\text{R+}\frac{\text{R}}{\text{3}}\right)\text{R}}{\left(\text{R+}\frac{\text{R}}{\text{3}}\right)\text{+R}}\right]\text{×2=}\frac{\text{8R}}{\text{7}}$