diff --git a/scripts/bench-cli-startup.ts b/scripts/bench-cli-startup.ts
index 1d5b8adb345..d28d06a103d 100644
--- a/scripts/bench-cli-startup.ts
+++ b/scripts/bench-cli-startup.ts
@@ -533,7 +533,12 @@ function parsePresets(raw: string | undefined): string[] {
 function resolveCases(options: { presets: string[]; caseIds: string[] }): CommandCase[] {
   const byId = new Map(COMMAND_CASES.map((commandCase) => [commandCase.id, commandCase]));
   if (options.caseIds.length > 0) {
+    const seenIds = new Set<string>();
     return options.caseIds.map((id) => {
+      if (seenIds.has(id)) {
+        throw new Error(`Duplicate --case "${id}"`);
+      }
+      seenIds.add(id);
       const commandCase = byId.get(id);
       if (!commandCase) {
         throw new Error(`Unknown --case "${id}"`);
diff --git a/scripts/bench-gateway-restart.ts b/scripts/bench-gateway-restart.ts
index 8e59757bfce..54adeed4313 100644
--- a/scripts/bench-gateway-restart.ts
+++ b/scripts/bench-gateway-restart.ts
@@ -337,8 +337,13 @@ function resolveCases(caseIds: string[]): GatewayBenchCase[] {
   if (caseIds.length === 0) {
     return [GATEWAY_CASES[0]];
   }
+  const seenIds = new Set<string>();
   const byId = new Map(GATEWAY_CASES.map((benchCase) => [benchCase.id, benchCase]));
   return caseIds.map((id) => {
+    if (seenIds.has(id)) {
+      throw new CliArgumentError(`Duplicate --case "${id}"`);
+    }
+    seenIds.add(id);
     const benchCase = byId.get(id);
     if (!benchCase) {
       throw new Error(`Unknown --case "${id}"`);
diff --git a/test/scripts/bench-cli-startup.test.ts b/test/scripts/bench-cli-startup.test.ts
index 10a8cc24ae4..97be9c335c8 100644
--- a/test/scripts/bench-cli-startup.test.ts
+++ b/test/scripts/bench-cli-startup.test.ts
@@ -57,6 +57,31 @@ describe("bench-cli-startup", () => {
     expect(result.stderr).not.toContain("\n    at ");
   });
 
+  it("rejects duplicate benchmark cases before running benchmarks", () => {
+    const result = spawnSync(
+      process.execPath,
+      [
+        "--import",
+        "tsx",
+        "scripts/bench-cli-startup.ts",
+        "--case",
+        "version",
+        "--case",
+        "version",
+      ],
+      {
+        cwd: join(__dirname, "../.."),
+        encoding: "utf8",
+      },
+    );
+
+    expect(result.status).toBe(1);
+    expect(result.stdout).toBe("");
+    expect(result.stderr.trim()).toBe('Duplicate --case "version"');
+    expect(result.stderr).not.toContain("Node.js");
+    expect(result.stderr).not.toContain("\n    at ");
+  });
+
   it.runIf(process.platform !== "win32")(
     "cleans timed-out benchmark process groups when the leader exits first",
     () => {
diff --git a/test/scripts/bench-gateway-restart.test.ts b/test/scripts/bench-gateway-restart.test.ts
index 35a1b421cda..47ead435237 100644
--- a/test/scripts/bench-gateway-restart.test.ts
+++ b/test/scripts/bench-gateway-restart.test.ts
@@ -91,6 +91,9 @@ describe("gateway restart benchmark script", () => {
       "--output requires a value",
     );
     expect(() => testing.parseOptions(["--case"])).toThrow("--case requires a value");
+    expect(() =>
+      testing.parseOptions(["--case", "skipChannels", "--case", "skipChannels"]),
+    ).toThrow('Duplicate --case "skipChannels"');
     expect(() => testing.parseOptions(["--restarts", "--runs", "1"])).toThrow(
       "--restarts requires a value",
     );
@@ -117,6 +120,34 @@ describe("gateway restart benchmark script", () => {
     expect(result.stderr).not.toContain("\n    at ");
   });
 
+  it("reports duplicate benchmark cases without a stack trace", () => {
+    const result = spawnSync(
+      process.execPath,
+      [
+        "--import",
+        "tsx",
+        "scripts/bench-gateway-restart.ts",
+        "--case",
+        "skipChannels",
+        "--case",
+        "skipChannels",
+      ],
+      {
+        cwd: process.cwd(),
+        encoding: "utf8",
+        env: {
+          ...process.env,
+          NODE_NO_WARNINGS: "1",
+        },
+      },
+    );
+
+    expect(result.status).toBe(1);
+    expect(result.stdout).toBe("");
+    expect(result.stderr.trim()).toBe('Duplicate --case "skipChannels"');
+    expect(result.stderr).not.toContain("\n    at ");
+  });
+
   it("guards the SIGUSR1 restart benchmark on Windows", () => {
     expect(() => testing.ensureSupportedRestartPlatform("linux")).not.toThrow();
     expect(() => testing.ensureSupportedRestartPlatform("darwin")).not.toThrow();