diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 77e182729e..0f562600c5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -61,10 +61,14 @@ jobs:
       - run: npm ci
       - run: npm run quality:collect
       # Catraca: falha se qualquer métrica regredir vs quality-baseline.json (commitado).
-      # Hoje: contagem de warnings do ESLint. Fases 3/4 estendem com duplicação,
-      # tamanho de arquivo e cobertura (esta última lida do coverage mergeado).
+      # Hoje: contagem de warnings do ESLint. Fase 4 estende com cobertura (lida do
+      # coverage mergeado). Tamanho de arquivo e duplicação têm gates dedicados.
       - name: Ratchet check
         run: node scripts/quality/check-quality-ratchet.mjs --summary .artifacts/quality-ratchet.md
+      # Catraca de duplicação (jscpd@4 sobre src+open-sse). Roda neste job (paralelo)
+      # para não pesar no caminho crítico do lint.
+      - name: Duplication ratchet
+        run: npm run check:duplication
       - name: Append summary
         if: always()
         run: cat .artifacts/quality-ratchet.md >> "$GITHUB_STEP_SUMMARY"
diff --git a/duplication-baseline.json b/duplication-baseline.json
new file mode 100644
index 0000000000..ccbfa72e13
--- /dev/null
+++ b/duplication-baseline.json
@@ -0,0 +1,4 @@
+{
+  "_comment": "Catraca de duplicacao (check-duplication.mjs, jscpd@4 sobre src+open-sse, min-tokens 50). So pode cair. --update ratcheta.",
+  "percentage": 5.72
+}
diff --git a/package.json b/package.json
index 14c6dbddeb..e559f25474 100644
--- a/package.json
+++ b/package.json
@@ -117,6 +117,7 @@
     "check:openapi-routes": "node scripts/check/check-openapi-routes.mjs",
     "check:deps": "node scripts/check/check-deps.mjs",
     "check:file-size": "node scripts/check/check-file-size.mjs",
+    "check:duplication": "node scripts/check/check-duplication.mjs",
     "quality:collect": "node scripts/quality/collect-metrics.mjs",
     "quality:ratchet": "node scripts/quality/check-quality-ratchet.mjs",
     "quality:gate": "npm run quality:collect && npm run quality:ratchet",
diff --git a/scripts/check/check-duplication.mjs b/scripts/check/check-duplication.mjs
new file mode 100644
index 0000000000..7843f16e88
--- /dev/null
+++ b/scripts/check/check-duplication.mjs
@@ -0,0 +1,63 @@
+#!/usr/bin/env node
+// scripts/check/check-duplication.mjs
+// Catraca de duplicação de código. Roda jscpd@4 (PINADO — o v5 é um rewrite Rust com
+// CLI/JSON incompatíveis) sobre src+open-sse e compara a % atual contra um baseline
+// congelado (duplication-baseline.json). Falha se a duplicação SUBIR. Ataca a assinatura
+// nº1 de slop de IA (GitClear 2026: duplicação 4-8x na era IA) — no nosso caso, o
+// copy-paste dos executors (48/50 sobrescrevem execute() inteiro). --update ratcheta.
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { execFileSync } from "node:child_process";
+import { pathToFileURL } from "node:url";
+
+const ROOT = process.cwd();
+const BASELINE_PATH = path.resolve(
+  process.argv.includes("--baseline")
+    ? process.argv[process.argv.indexOf("--baseline") + 1]
+    : path.join(ROOT, "duplication-baseline.json")
+);
+const UPDATE = process.argv.includes("--update");
+const EPS = 0.05; // tolerância de ruído de float (jscpd é determinístico; isto é margem)
+const JSCPD_ARGS = ["jscpd@4", "src", "open-sse", "--reporters", "json", "--silent", "--min-tokens", "50", "--ignore", "**/*.test.ts,**/*.test.tsx,**/__tests__/**"];
+
+/** Avalia a % atual contra o baseline. */
+export function evaluateDuplication(current, baseline, eps = EPS) {
+  return {
+    regressed: current > baseline + eps,
+    improved: current < baseline - eps,
+  };
+}
+
+function measureDuplicationPct() {
+  const out = fs.mkdtempSync(path.join(os.tmpdir(), "jscpd-"));
+  execFileSync("npx", ["--yes", ...JSCPD_ARGS, "--output", out], { stdio: "ignore" });
+  const report = JSON.parse(fs.readFileSync(path.join(out, "jscpd-report.json"), "utf8"));
+  return report.statistics.total.percentage;
+}
+
+function main() {
+  if (!fs.existsSync(BASELINE_PATH)) {
+    console.error(`[duplication] FAIL — ${path.basename(BASELINE_PATH)} ausente.`);
+    process.exit(2);
+  }
+  const baseline = JSON.parse(fs.readFileSync(BASELINE_PATH, "utf8"));
+  const current = measureDuplicationPct();
+  const { regressed, improved } = evaluateDuplication(current, baseline.percentage, EPS);
+
+  if (UPDATE && improved) {
+    baseline.percentage = current;
+    fs.writeFileSync(BASELINE_PATH, JSON.stringify(baseline, null, 2) + "\n");
+    console.log(`[duplication] baseline ratcheado: ${current}% (era ${baseline.percentage}%)`);
+  }
+  if (regressed) {
+    console.error(
+      `[duplication] REGRESSÃO — ${current}% > baseline ${baseline.percentage}% (+${EPS} tolerância)\n` +
+        `  → extraia o trecho duplicado (helper compartilhado) ou ajuste duplication-baseline.json com justificativa.`
+    );
+    process.exit(1);
+  }
+  console.log(`[duplication] OK — ${current}% (baseline ${baseline.percentage}%)`);
+}
+
+if (import.meta.url === pathToFileURL(process.argv[1] || "").href) main();
diff --git a/tests/unit/check-duplication.test.ts b/tests/unit/check-duplication.test.ts
new file mode 100644
index 0000000000..be4939e1e8
--- /dev/null
+++ b/tests/unit/check-duplication.test.ts
@@ -0,0 +1,24 @@
+import { test } from "node:test";
+import assert from "node:assert";
+import { evaluateDuplication } from "../../scripts/check/check-duplication.mjs";
+
+const EPS = 0.05;
+
+test("equal to baseline passes", () => {
+  assert.equal(evaluateDuplication(5.72, 5.72, EPS).regressed, false);
+});
+
+test("within epsilon passes (float noise)", () => {
+  assert.equal(evaluateDuplication(5.74, 5.72, EPS).regressed, false);
+});
+
+test("meaningful increase is a regression", () => {
+  const r = evaluateDuplication(5.9, 5.72, EPS);
+  assert.equal(r.regressed, true);
+});
+
+test("a decrease is an improvement (ratchet down)", () => {
+  const r = evaluateDuplication(5.0, 5.72, EPS);
+  assert.equal(r.regressed, false);
+  assert.equal(r.improved, true);
+});