diff --git a/scripts/dev/gateway-smoke.ts b/scripts/dev/gateway-smoke.ts
index ab61d0e1ae1f..c0df403d4fdc 100644
--- a/scripts/dev/gateway-smoke.ts
+++ b/scripts/dev/gateway-smoke.ts
@@ -49,9 +49,14 @@ function usage(): string {
 }
 
 function validateArgs(argv: readonly string[]): void {
+  const seen = new Set<string>();
   for (let index = 0; index < argv.length; index += 1) {
     const arg = argv[index] ?? "";
     if (BOOLEAN_FLAGS.has(arg)) {
+      if (seen.has(arg)) {
+        throw new GatewaySmokeArgError(`${arg} was provided more than once`);
+      }
+      seen.add(arg);
       continue;
     }
     if (VALUE_FLAGS.has(arg)) {
@@ -59,6 +64,10 @@ function validateArgs(argv: readonly string[]): void {
       if (!value || value.startsWith("-")) {
         throw new GatewaySmokeArgError(`${arg} requires a value`);
       }
+      if (seen.has(arg)) {
+        throw new GatewaySmokeArgError(`${arg} was provided more than once`);
+      }
+      seen.add(arg);
       index += 1;
       continue;
     }
diff --git a/test/e2e/qa-lab/runtime/gateway-smoke.e2e.test.ts b/test/e2e/qa-lab/runtime/gateway-smoke.e2e.test.ts
index 56acfd2dff78..86cfe65e1f0b 100644
--- a/test/e2e/qa-lab/runtime/gateway-smoke.e2e.test.ts
+++ b/test/e2e/qa-lab/runtime/gateway-smoke.e2e.test.ts
@@ -223,6 +223,33 @@ describe("gateway-smoke", () => {
     }
   });
 
+  it("rejects duplicate CLI args before connecting", () => {
+    for (const [flag, args] of [
+      [
+        "--url",
+        ["--url", "ws://127.0.0.1:9", "--url", "ws://127.0.0.1:10", "--token", "token"],
+      ],
+      [
+        "--token",
+        ["--url", "ws://127.0.0.1:9", "--token", "one", "--token", "two"],
+      ],
+      ["--help", ["--help", "--help"]],
+    ] as const) {
+      const result = spawnSync(
+        process.execPath,
+        ["--import", "tsx", "scripts/dev/gateway-smoke.ts", ...args],
+        {
+          cwd: process.cwd(),
+          encoding: "utf8",
+        },
+      );
+
+      expect(result.status).toBe(1);
+      expect(result.stdout).toBe("");
+      expect(result.stderr.trim()).toBe(`${flag} was provided more than once`);
+    }
+  });
+
   it("passes against a loopback gateway websocket using the real client", async () => {
     const stdout: string[] = [];
     const stderr: string[] = [];