undistinguishable

From un- +‎ distinguishable.

• (UK) IPA^(key): /ˌʌndɪˈstɪŋɡwɪʃəb(ə)l/

undistinguishable (comparative more undistinguishable, superlative most undistinguishable)

1. Imperceptible; indistinct.
2. Indistinguishable.
   • 1869, Alfred Russel Wallace, The Malay Archipelago, volume I, London: Macmillan and Co., page 297:

     Here I obtained the rare and beautiful swallow-tail butterflies, Papilio aenomaus and P. liris; the males of which are quite unlike each other, and belong in fact to distinct sections of the genus, while the females are so much alike that they are undistinguishable on the wing, and to an uneducated eye equally so in the cabinet.