WordNet sense 1 (being the exact same one; not any other:):

very, selfsame

WordNet sense 2 (same in identity):

same

WordNet sense 3 (exactly alike; incapable of being perceived as different):

indistinguishable

WordNet sense 4 (invariant with respect to direction):

isotropic, isotropous

WordNet sense 5 (coinciding exactly when superimposed):

superposable

WordNet sense 6 ((of twins) derived from a single egg or ovum):

monovular