diff --git a/scripts/security-validation-evidence.mjs b/scripts/security-validation-evidence.mjs
old mode 100644
new mode 100755
index 774cb32f8..78c524bad
--- a/scripts/security-validation-evidence.mjs
+++ b/scripts/security-validation-evidence.mjs
@@ -3,6 +3,7 @@ import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { dirname, join } from "node:path";
 import { TextDecoder } from "node:util";
 import { evaluateSecurityChecklistText, evaluateSecurityEvidence } from "./lib/security-checklist.mjs";
+import { hasDuplicateJsonObjectKeys } from "./normalize-commercial-readiness-evidence.mjs";
 
 const generatedAt = new Date().toISOString();
 const checklistPath = process.env.NOEMA_SECURITY_CHECKLIST_PATH || "docs/security-validation-checklist.md";
@@ -52,6 +53,9 @@ function readJson(path) {
     return { ok: false, reason: "invalid_utf8", path, error: error.message };
   }
   try {
+    if (hasDuplicateJsonObjectKeys(text)) {
+      return { ok: false, reason: "duplicate_keys", path };
+    }
     return { ok: true, path, value: JSON.parse(text) };
   } catch (error) {
     return { ok: false, reason: "invalid_json", path, error: error.message };
diff --git a/test/security-validation-evidence-duplicate-json.test.ts b/test/security-validation-evidence-duplicate-json.test.ts
new file mode 100644
index 000000000..75780c6a2
--- /dev/null
+++ b/test/security-validation-evidence-duplicate-json.test.ts
@@ -0,0 +1,49 @@
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { spawnSync } from "node:child_process";
+import { describe, expect, it } from "vitest";
+
+function runSecurityEvidence(checklistPath: string, evidencePath: string, auditPath: string) {
+  return spawnSync(process.execPath, ["scripts/security-validation-evidence.mjs"], {
+    cwd: process.cwd(),
+    env: {
+      ...process.env,
+      NOEMA_SECURITY_CHECKLIST_PATH: checklistPath,
+      NOEMA_SECURITY_EVIDENCE_PATH: evidencePath,
+      NOEMA_SECURITY_AUDIT_PATH: auditPath,
+    },
+    encoding: "utf8",
+  });
+}
+
+describe("security validation evidence JSON integrity", () => {
+  it("rejects duplicate decoded keys instead of accepting JSON last-key-wins evidence", () => {
+    const temp = mkdtempSync(join(tmpdir(), "noema-security-evidence-duplicate-json-"));
+    try {
+      const checklistPath = join(temp, "checklist.md");
+      const evidencePath = join(temp, "security-validation-evidence.json");
+      const auditPath = join(temp, "audit", "security-validation-audit.json");
+      writeFileSync(checklistPath, "- [x] release gate passed\n- [x] smoke evidence reviewed\n", "utf8");
+      writeFileSync(
+        evidencePath,
+        '{"checklist_path":"docs/security-validation-checklist.md","updated_at":"2026-07-02","owner":"replace-with-security-owner","ow\\u006eer":"security","source_documents":["security/noema-prod-review.md"],"validation_artifacts":["artifacts/security/release-verify.log"]}',
+        "utf8",
+      );
+
+      const result = runSecurityEvidence(checklistPath, evidencePath, auditPath);
+
+      expect(result.status).toBe(1);
+      expect(result.stdout).toContain("security-validation-evidence: FAIL");
+      const audit = JSON.parse(readFileSync(auditPath, "utf8"));
+      expect(audit.passed).toBe(false);
+      expect(
+        audit.checks.find(
+          (check: { name: string }) => check.name === "security validation evidence present",
+        )?.details?.reason,
+      ).toBe("duplicate_keys");
+    } finally {
+      rmSync(temp, { recursive: true, force: true });
+    }
+  });
+});