∀ {α : Type u_1} [inst : BEq α] (head : α) (tail : List α),
  (tail.Nodup → ∀ (i : ℕ) (h : i < tail.length), List.idxOf tail[i] tail = i) →
    (head :: tail).Nodup →
      ∀ (i : ℕ) (h : i < (head :: tail).length) (h_2 : ¬i = 0),
        -1 * ↑i + 1 ≤ 0 → List.idxOf tail[i - 1] tail < tail.length