Uniquify Identifiers
Brachylog, 8 bytes
{|∧ℕ₁}ᵐ≠

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Explanation
{    }ᵐ     Map on the Input:
              Input = Output…
 |            …or…
  ∧ℕ₁         …Output is in [1,+inf)
       ≠    All elements of the output must be different
            (Implicit labeling)