diff --git a/scripts/bench-test-changed.mjs b/scripts/bench-test-changed.mjs
index e8104791a616..a7ff040dce20 100644
--- a/scripts/bench-test-changed.mjs
+++ b/scripts/bench-test-changed.mjs
@@ -31,7 +31,9 @@ function positiveIntegerFlag(flag, key) {
         throw new Error(`${flag} requires a value`);
       }
       return {
+        flag,
         nextIndex: index + 1,
+        repeatable: false,
         apply(target) {
           target[key] = parsePositiveInteger(rawValue, flag);
         },
diff --git a/scripts/lib/arg-utils.mjs b/scripts/lib/arg-utils.mjs
index 76ef2bc27c42..f57fdf00fe2f 100644
--- a/scripts/lib/arg-utils.mjs
+++ b/scripts/lib/arg-utils.mjs
@@ -143,7 +143,9 @@ export function stringFlag(flag, key, options = {}) {
         return null;
       }
       return {
+        flag,
         nextIndex: option.nextIndex,
+        repeatable: false,
         apply(target) {
           target[key] = option.value;
         },
@@ -161,7 +163,9 @@ export function stringListFlag(flag, key, options = {}) {
         return null;
       }
       return {
+        flag,
         nextIndex: option.nextIndex,
+        repeatable: true,
         apply(target) {
           target[key] ??= [];
           target[key].push(option.value);
@@ -171,7 +175,7 @@ export function stringListFlag(flag, key, options = {}) {
   };
 }
 
-function createAssignedValueFlag(consumeOption) {
+function createAssignedValueFlag(flag, consumeOption) {
   return {
     consume(argv, index, args) {
       const option = consumeOption(argv, index, args);
@@ -179,7 +183,9 @@ function createAssignedValueFlag(consumeOption) {
         return null;
       }
       return {
+        flag,
         nextIndex: option.nextIndex,
+        repeatable: false,
         apply(target) {
           target[option.key] = option.value;
         },
@@ -190,7 +196,7 @@ function createAssignedValueFlag(consumeOption) {
 
 /** Create a flag spec that parses and assigns a safe integer value. */
 export function intFlag(flag, key, options) {
-  return createAssignedValueFlag((argv, index) => {
+  return createAssignedValueFlag(flag, (argv, index) => {
     const option = consumeIntFlag(argv, index, flag, options);
     return option ? { ...option, key } : null;
   });
@@ -198,7 +204,7 @@ export function intFlag(flag, key, options) {
 
 /** Create a flag spec that parses and assigns a finite floating-point value. */
 export function floatFlag(flag, key, options) {
-  return createAssignedValueFlag((argv, index) => {
+  return createAssignedValueFlag(flag, (argv, index) => {
     const option = consumeFloatFlag(argv, index, flag, options);
     return option ? { ...option, key } : null;
   });
@@ -212,7 +218,9 @@ export function booleanFlag(flag, key, value = true) {
         return null;
       }
       return {
+        flag,
         nextIndex: index,
+        repeatable: false,
         apply(target) {
           target[key] = value;
         },
@@ -224,6 +232,7 @@ export function booleanFlag(flag, key, value = true) {
 /** Apply flag specs to argv and return the mutated parsed args object. */
 export function parseFlagArgs(argv, args, specs, options = {}) {
   const ignoreDoubleDash = options.ignoreDoubleDash ?? true;
+  const seenFlags = new Set();
   for (let i = 0; i < argv.length; i += 1) {
     const arg = argv[i];
     if (arg === "--" && ignoreDoubleDash) {
@@ -235,6 +244,15 @@ export function parseFlagArgs(argv, args, specs, options = {}) {
       if (!option) {
         continue;
       }
+      if (typeof option.flag !== "string" || !option.flag) {
+        throw new Error("parseFlagArgs specs must declare a flag for consumed options");
+      }
+      if (option.repeatable !== true) {
+        if (seenFlags.has(option.flag)) {
+          throw new Error(`${option.flag} was provided more than once`);
+        }
+        seenFlags.add(option.flag);
+      }
       option.apply(args);
       i = option.nextIndex;
       handled = true;
diff --git a/scripts/lib/budget-number-args.mjs b/scripts/lib/budget-number-args.mjs
index 2401c957d3a8..03cbe9f2ed6f 100644
--- a/scripts/lib/budget-number-args.mjs
+++ b/scripts/lib/budget-number-args.mjs
@@ -29,7 +29,9 @@ export function budgetFloatFlag(flag, key) {
         throw new Error(`${flag} requires a value`);
       }
       return {
+        flag,
         nextIndex: index + 1,
+        repeatable: false,
         apply(target) {
           const parsed = parseBudgetNumber(value, flag);
           if (parsed === null) {
diff --git a/test/scripts/arg-utils.test.ts b/test/scripts/arg-utils.test.ts
index 45b5f7800301..ce369a1cbc35 100644
--- a/test/scripts/arg-utils.test.ts
+++ b/test/scripts/arg-utils.test.ts
@@ -1,6 +1,7 @@
 // Arg Utils tests cover arg utils script behavior.
 import { describe, expect, it } from "vitest";
 import {
+  booleanFlag,
   floatFlag,
   intFlag,
   parseFlagArgs,
@@ -43,6 +44,44 @@ describe("scripts/lib/arg-utils parseFlagArgs", () => {
     expect(parsed.match).toEqual(["alpha", "beta"]);
   });
 
+  it("rejects duplicate single-value flags", () => {
+    expect(() =>
+      parseFlagArgs(["--label", "first", "--label=second"], { label: "" }, [
+        stringFlag("--label", "label"),
+      ]),
+    ).toThrow("--label was provided more than once");
+    expect(() =>
+      parseFlagArgs(["--limit", "1", "--limit=2"], { limit: 10 }, [
+        intFlag("--limit", "limit", { min: 1 }),
+      ]),
+    ).toThrow("--limit was provided more than once");
+    expect(() =>
+      parseFlagArgs(["--json", "--json"], { json: false }, [booleanFlag("--json", "json")]),
+    ).toThrow("--json was provided more than once");
+  });
+
+  it("requires custom specs to declare consumed flags", () => {
+    expect(() =>
+      parseFlagArgs(
+        ["--custom"],
+        {},
+        [
+          {
+            consume(argv, index) {
+              if (argv[index] !== "--custom") {
+                return null;
+              }
+              return {
+                nextIndex: index,
+                apply() {},
+              };
+            },
+          },
+        ],
+      ),
+    ).toThrow("parseFlagArgs specs must declare a flag for consumed options");
+  });
+
   it("rejects missing string flag values before consuming the next option", () => {
     expect(() =>
       parseFlagArgs(["--base", "--head", "HEAD"], { base: "origin/main", head: "HEAD" }, [
diff --git a/test/scripts/bench-test-changed.test.ts b/test/scripts/bench-test-changed.test.ts
index 6d4572918ef8..3486e2e6a356 100644
--- a/test/scripts/bench-test-changed.test.ts
+++ b/test/scripts/bench-test-changed.test.ts
@@ -87,6 +87,15 @@ describe("bench-test-changed script", () => {
     expect(nextFlag.stderr).not.toContain("at ");
   });
 
+  it("rejects duplicate max worker values before inspecting git state", () => {
+    const result = runBenchTestChanged(["--max-workers", "2", "--max-workers", "3"]);
+
+    expect(result.status).toBe(1);
+    expect(result.stdout).toBe("");
+    expect(result.stderr).toContain("--max-workers was provided more than once");
+    expect(result.stderr).not.toContain("at ");
+  });
+
   it("rejects unknown options before collecting changed paths", () => {
     const result = runBenchTestChanged(["--max-worker", "4"]);
 
diff --git a/test/scripts/qa-report-cli.test.ts b/test/scripts/qa-report-cli.test.ts
index 42d910aba247..d7495c1aadef 100644
--- a/test/scripts/qa-report-cli.test.ts
+++ b/test/scripts/qa-report-cli.test.ts
@@ -43,6 +43,20 @@ describe("QA report source CLIs", () => {
     expectNoNodeStack(result.stderr);
   });
 
+  it("rejects duplicate QA coverage artifact destinations without a Node stack trace", () => {
+    const result = runSourceScript(
+      "scripts/qa-coverage-report.ts",
+      "--output",
+      ".artifacts/first.md",
+      "--output=.artifacts/second.md",
+    );
+
+    expect(result.status).toBe(1);
+    expect(result.stdout).toBe("");
+    expect(result.stderr.trim()).toBe("--output was provided more than once");
+    expectNoNodeStack(result.stderr);
+  });
+
   it("reports unknown QA parity options without a Node stack trace", () => {
     const result = runSourceScript("scripts/qa-parity-report.ts", "--wat");
 
@@ -52,6 +66,24 @@ describe("QA report source CLIs", () => {
     expectNoNodeStack(result.stderr);
   });
 
+  it("rejects duplicate QA parity artifact destinations without a Node stack trace", () => {
+    const result = runSourceScript(
+      "scripts/qa-parity-report.ts",
+      "--candidate-summary",
+      "candidate.json",
+      "--baseline-summary",
+      "baseline.json",
+      "--output-dir",
+      ".artifacts/first",
+      "--output-dir=.artifacts/second",
+    );
+
+    expect(result.status).toBe(1);
+    expect(result.stdout).toBe("");
+    expect(result.stderr.trim()).toBe("--output-dir was provided more than once");
+    expectNoNodeStack(result.stderr);
+  });
+
   it("reports missing QA parity inputs without a Node stack trace", () => {
     const result = runSourceScript("scripts/qa-parity-report.ts");
 
diff --git a/test/scripts/test-perf-budget.test.ts b/test/scripts/test-perf-budget.test.ts
index 867fb4d4f1e6..97b11a8f5542 100644
--- a/test/scripts/test-perf-budget.test.ts
+++ b/test/scripts/test-perf-budget.test.ts
@@ -85,6 +85,9 @@ describe("test perf budget script", () => {
     expect(() => testing.parseArgs(["--max-regression-pct", "-h"], {})).toThrow(
       "--max-regression-pct requires a value",
     );
+    expect(() => testing.parseArgs(["--max-wall-ms", "1000", "--max-wall-ms", "2000"], {})).toThrow(
+      "--max-wall-ms was provided more than once",
+    );
   });
 
   it("requires timed file evidence in the Vitest JSON report", () => {