theorem extracted_formal_statement_339.{u_1, w, u} {Ω : Type u} [inst : MeasurableSpace Ω] {Z : Type w} {n : ℕ}
  {ι : Type u_1} {f : ι → Z → ℝ} {μ : Measure Ω} [inst_1 : IsProbabilityMeasure μ] {X : Ω → Z} [inst_2 : Nonempty ι]
  [inst_3 : Countable ι] (hf : ∀ (i : ι), Measurable (f i ∘ X)) {b : ℝ} (hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b)
  (h_original_goal :
    ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
          (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.1 k))|)
            x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) +
        ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
          (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.2 k))|)
            x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) =
      2 *
        ∫ (x : Fin n → Ω),
          (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k))|)
            x ∂Measure.pi fun x => μ) :
  ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
        (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.1 k))|)
          x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) +
      ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
        (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.2 k))|)
          x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) =
    (∫ (x : Fin n → Ω),
        (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k))|) x ∂Measure.pi fun x => μ) +
      ∫ (x : Fin n → Ω),
        (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k))|)
          x ∂Measure.pi fun x => μ := sorry

State before:
Ω : Type u
inst✝³ : MeasurableSpace Ω
Z : Type w
n : ℕ
ι : Type u_1
f : ι → Z → ℝ
μ : Measure Ω
inst✝² : IsProbabilityMeasure μ
X : Ω → Z
inst✝¹ : Nonempty ι
inst✝ : Countable ι
hf : ∀ (i : ι), Measurable (f i ∘ X)
b : ℝ
hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
⊢ ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
        (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.1 k))|)
          x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) +
      ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
        (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.2 k))|)
          x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) =
    2 *
      ∫ (x : Fin n → Ω),
        (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k))|) x ∂Measure.pi fun x => μ

Tactic: rw [two_mul]

State after:
[Ω : Type u
 inst✝³ : MeasurableSpace Ω
 Z : Type w
 n : ℕ
 ι : Type u_1
 f : ι → Z → ℝ
 μ : Measure Ω
 inst✝² : IsProbabilityMeasure μ
 X : Ω → Z
 inst✝¹ : Nonempty ι
 inst✝ : Countable ι
 hf : ∀ (i : ι), Measurable (f i ∘ X)
 b : ℝ
 hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
 ⊢ ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
         (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.1 k))|)
           x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) +
       ∫ (x : (Fin n → Ω) × (Fin n → Ω)),
         (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω.2 k))|)
           x ∂(Measure.pi fun x => μ).prod (Measure.pi fun x => μ) =
     (∫ (x : Fin n → Ω),
         (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k))|) x ∂Measure.pi fun x => μ) +
       ∫ (x : Fin n → Ω),
         (fun ω => (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k))|) x ∂Measure.pi fun x => μ]

Relevant lemmas:
two_mul : ∀ {α : Type u} [inst : NonAssocSemiring α] (n : α), 2 * n = n + n