diff --git a/scripts/dependency-changes-report.mjs b/scripts/dependency-changes-report.mjs
index 04f5f879688..0634461f9a0 100644
--- a/scripts/dependency-changes-report.mjs
+++ b/scripts/dependency-changes-report.mjs
@@ -249,38 +249,46 @@ export function parseArgs(argv) {
     jsonPath: null,
     markdownPath: null,
   };
+  const seen = new Set();
+  const setOnce = (flag, key, value) => {
+    if (seen.has(flag)) {
+      throw new Error(`${flag} was provided more than once.`);
+    }
+    seen.add(flag);
+    options[key] = value;
+  };
   for (let index = 0; index < argv.length; index += 1) {
     const arg = argv[index];
     if (arg === "--") {
       continue;
     }
     if (arg === "--root") {
-      options.rootDir = readRequiredValue(argv, index, "--root");
+      setOnce(arg, "rootDir", readRequiredValue(argv, index, "--root"));
       index += 1;
       continue;
     }
     if (arg === "--base-ref") {
-      options.baseRef = readRequiredValue(argv, index, "--base-ref");
+      setOnce(arg, "baseRef", readRequiredValue(argv, index, "--base-ref"));
       index += 1;
       continue;
     }
     if (arg === "--base-lockfile") {
-      options.baseLockfile = readRequiredValue(argv, index, "--base-lockfile");
+      setOnce(arg, "baseLockfile", readRequiredValue(argv, index, "--base-lockfile"));
       index += 1;
       continue;
     }
     if (arg === "--head-lockfile") {
-      options.headLockfile = readRequiredValue(argv, index, "--head-lockfile");
+      setOnce(arg, "headLockfile", readRequiredValue(argv, index, "--head-lockfile"));
       index += 1;
       continue;
     }
     if (arg === "--json") {
-      options.jsonPath = readRequiredValue(argv, index, "--json");
+      setOnce(arg, "jsonPath", readRequiredValue(argv, index, "--json"));
       index += 1;
       continue;
     }
     if (arg === "--markdown") {
-      options.markdownPath = readRequiredValue(argv, index, "--markdown");
+      setOnce(arg, "markdownPath", readRequiredValue(argv, index, "--markdown"));
       index += 1;
       continue;
     }
@@ -289,6 +297,9 @@ export function parseArgs(argv) {
   if (!options.baseRef && !options.baseLockfile) {
     throw new Error("Expected --base-ref <git-ref> or --base-lockfile <path>.");
   }
+  if (options.baseRef && options.baseLockfile) {
+    throw new Error("Use either --base-ref or --base-lockfile, not both.");
+  }
   return options;
 }
 
diff --git a/scripts/dependency-ownership-surface-report.mjs b/scripts/dependency-ownership-surface-report.mjs
index 7775990b233..47ed49f0a51 100644
--- a/scripts/dependency-ownership-surface-report.mjs
+++ b/scripts/dependency-ownership-surface-report.mjs
@@ -412,6 +412,14 @@ export function parseArgs(argv) {
     jsonPath: null,
     markdownPath: null,
   };
+  const seen = new Set();
+  const setOnce = (flag, key, value) => {
+    if (seen.has(flag)) {
+      throw new Error(`${flag} was provided more than once.`);
+    }
+    seen.add(flag);
+    options[key] = value;
+  };
   for (let index = 0; index < argv.length; index += 1) {
     const arg = argv[index];
     if (arg === "--") {
@@ -422,6 +430,10 @@ export function parseArgs(argv) {
       continue;
     }
     if (arg === "--json") {
+      if (seen.has(arg)) {
+        throw new Error(`${arg} was provided more than once.`);
+      }
+      seen.add(arg);
       options.asJson = true;
       if (argv[index + 1] && !argv[index + 1].startsWith("-")) {
         options.jsonPath = argv[++index];
@@ -429,7 +441,7 @@ export function parseArgs(argv) {
       continue;
     }
     if (arg === "--markdown") {
-      options.markdownPath = readArtifactPath(argv, index, arg);
+      setOnce(arg, "markdownPath", readArtifactPath(argv, index, arg));
       index += 1;
       continue;
     }
diff --git a/test/scripts/dependency-changes-report.test.ts b/test/scripts/dependency-changes-report.test.ts
index ef6218ce3c5..fbead670201 100644
--- a/test/scripts/dependency-changes-report.test.ts
+++ b/test/scripts/dependency-changes-report.test.ts
@@ -88,6 +88,24 @@ describe("dependency-changes-report", () => {
     }
   });
 
+  it("rejects duplicate and conflicting dependency report inputs", () => {
+    for (const flag of [
+      "--root",
+      "--base-ref",
+      "--base-lockfile",
+      "--head-lockfile",
+      "--json",
+      "--markdown",
+    ]) {
+      expect(() => parseArgs(["--base-ref", "main", flag, "one", flag, "two"])).toThrow(
+        `${flag} was provided more than once.`,
+      );
+    }
+    expect(() => parseArgs(["--base-ref", "main", "--base-lockfile", "base-lock.yaml"])).toThrow(
+      "Use either --base-ref or --base-lockfile, not both.",
+    );
+  });
+
   it("reports CLI argument errors without a Node stack trace", () => {
     const missingBase = runCli();
     expect(missingBase.status).toBe(1);
diff --git a/test/scripts/dependency-ownership-surface-report.test.ts b/test/scripts/dependency-ownership-surface-report.test.ts
index 63d469a73a9..383d0b1b19f 100644
--- a/test/scripts/dependency-ownership-surface-report.test.ts
+++ b/test/scripts/dependency-ownership-surface-report.test.ts
@@ -57,6 +57,15 @@ describe("parseArgs", () => {
     });
     expect(() => parseArgs(["--json", "-h"])).toThrow("Unsupported argument: -h");
   });
+
+  it("rejects duplicate report artifact options", () => {
+    expect(() => parseArgs(["--json", "first.json", "--json", "second.json"])).toThrow(
+      "--json was provided more than once.",
+    );
+    expect(() =>
+      parseArgs(["--markdown", "first.md", "--markdown", "second.md"]),
+    ).toThrow("--markdown was provided more than once.");
+  });
 });
 
 describe("collectDependencyOwnershipSurfaceReport", () => {