selfsameness

/ˈsɛlfˌseɪmnəs/
noun
  1. The quality or state of being exactly the same; complete identity or uniformity.
    • The selfsameness of the twins' handwriting made it hard to tell who wrote the note.
    • The selfsameness of the two signatures proved the document was a forgery.
    • Philosophers have long debated the nature of selfsameness over time.