diff --git a/packages/normalization-core/src/error-coercion.test.ts b/packages/normalization-core/src/error-coercion.test.ts
index 64a665171b65..fc543fb6fafe 100644
--- a/packages/normalization-core/src/error-coercion.test.ts
+++ b/packages/normalization-core/src/error-coercion.test.ts
@@ -26,6 +26,27 @@ describe("formatErrorMessage", () => {
     );
   });
 
+  it("omits cause text the wrapper message already spells out", () => {
+    // Wrappers that embed the cause verbatim printed the whole sentence twice.
+    const parseFailure = new SyntaxError("JSON5: invalid character 'j' at 1:7");
+    const wrapped = new Error(`Failed to parse --file as JSON5: ${parseFailure.message}`, {
+      cause: parseFailure,
+    });
+    expect(format(wrapped)).toBe(
+      "Failed to parse --file as JSON5: JSON5: invalid character 'j' at 1:7",
+    );
+
+    // Codes keep their own segment even when the detail already names them.
+    const errno = Object.assign(
+      new Error("ENOENT: no such file or directory, open '/tmp/missing.json'"),
+      { code: "ENOENT" },
+    );
+    const notFound = new Error("--file not found: /tmp/missing.json.", { cause: errno });
+    expect(format(notFound)).toBe(
+      "--file not found: /tmp/missing.json. | ENOENT: no such file or directory, open '/tmp/missing.json' | ENOENT",
+    );
+  });
+
   it("formats status/code records and structured non-Error causes", () => {
     expect(format({ status: 500, code: "EPIPE" })).toBe("status=500 code=EPIPE");
     expect(format({ status: 404 })).toBe("status=404 code=unknown");
diff --git a/packages/normalization-core/src/error-coercion.ts b/packages/normalization-core/src/error-coercion.ts
index 6deeed1300f2..c96cc060e2b7 100644
--- a/packages/normalization-core/src/error-coercion.ts
+++ b/packages/normalization-core/src/error-coercion.ts
@@ -87,6 +87,17 @@ export function formatErrorMessage(value: unknown, options: FormatErrorMessageOp
       formatted += ` | ${message}`;
       seenMessages.add(message);
     };
+    // Wrappers routinely embed the cause verbatim ("failed to parse X: <cause.message>"),
+    // which exact-match dedupe misses, so the whole sentence prints twice. Codes stay on
+    // their own: a trailing bare code is this formatter's convention even when the detail
+    // already names it.
+    const appendCauseErrorMessage = (message: string | undefined): void => {
+      if (message && formatted.includes(message)) {
+        seenMessages.add(message);
+        return;
+      }
+      appendCauseMessage(message);
+    };
     if (options.includeCode) {
       const code = readProperty(value, "code");
       if (typeof code === "string" || typeof code === "number") {
@@ -96,7 +107,7 @@ export function formatErrorMessage(value: unknown, options: FormatErrorMessageOp
     while (cause && !seen.has(cause)) {
       seen.add(cause);
       if (cause instanceof Error) {
-        appendCauseMessage(cause.message);
+        appendCauseErrorMessage(cause.message);
         const code = readProperty(cause, "code");
         if (typeof code === "string" || typeof code === "number") {
           appendCauseMessage(String(code));