⊦ dups(x) ↔ (∃ a. a ∈ x ∧ #oc(a, x) ≠ 1)
Used simplifier rules: in-05: ⊦ a ∈ x → #oc(a, x) ≠ 0;% ⊦ 1 < m ↔ ¬(m = 0 ∨ m = 1); (s)% ⊦ m > n ↔ n < m; (s)