diff --git a/packages/core/src/config.ts b/packages/core/src/config.ts
index 528fbce5..69062d73 100644
--- a/packages/core/src/config.ts
+++ b/packages/core/src/config.ts
@@ -11,6 +11,7 @@ import {
 import glob from 'fast-glob';
 import { DEFAULT_CONFIG_NAME, DEFAULT_EXTENSIONS } from './constant';
 import type {
+  AutoExternal,
   Format,
   LibConfig,
   PkgJson,
@@ -21,7 +22,6 @@ import type {
   Syntax,
 } from './types';
 import { getDefaultExtension } from './utils/extension';
-import { composeAutoExternalConfig } from './utils/external';
 import {
   calcLongestCommonPath,
   color,
@@ -82,6 +82,60 @@ export async function loadConfig(
   return content as RslibConfig;
 }
 
+export const composeAutoExternalConfig = (options: {
+  autoExternal: AutoExternal;
+  pkgJson?: PkgJson;
+  userExternals?: NonNullable<RsbuildConfig['output']>['externals'];
+}): RsbuildConfig => {
+  const { autoExternal, pkgJson, userExternals } = options;
+
+  if (!autoExternal) {
+    return {};
+  }
+
+  if (!pkgJson) {
+    logger.warn(
+      'autoExternal configuration will not be applied due to read package.json failed',
+    );
+    return {};
+  }
+
+  const externalOptions = {
+    dependencies: true,
+    peerDependencies: true,
+    devDependencies: false,
+    ...(autoExternal === true ? {} : autoExternal),
+  };
+
+  // User externals configuration has higher priority than autoExternal
+  // eg: autoExternal: ['react'], user: output: { externals: { react: 'react-1' } }
+  // Only handle the case where the externals type is object, string / string[] does not need to be processed, other types are too complex.
+  const userExternalKeys =
+    userExternals &&
+    Object.prototype.toString.call(userExternals) === '[object Object]'
+      ? Object.keys(userExternals)
+      : [];
+
+  const externals = (
+    ['dependencies', 'peerDependencies', 'devDependencies'] as const
+  )
+    .reduce<string[]>((prev, type) => {
+      if (externalOptions[type]) {
+        return pkgJson[type] ? prev.concat(Object.keys(pkgJson[type]!)) : prev;
+      }
+      return prev;
+    }, [])
+    .filter((name) => !userExternalKeys.includes(name));
+
+  return externals.length
+    ? {
+        output: {
+          externals: Array.from(new Set(externals)),
+        },
+      }
+    : {};
+};
+
 export async function createInternalRsbuildConfig(): Promise<RsbuildConfig> {
   return defineRsbuildConfig({
     dev: {
diff --git a/packages/core/src/utils/external.ts b/packages/core/src/utils/external.ts
deleted file mode 100644
index c066fbb0..00000000
--- a/packages/core/src/utils/external.ts
+++ /dev/null
@@ -1,58 +0,0 @@
-import type { RsbuildConfig } from '@rsbuild/core';
-import type { PkgJson } from '../types';
-import type { AutoExternal } from '../types/config';
-import { logger } from './logger';
-
-export const composeAutoExternalConfig = (options: {
-  autoExternal: AutoExternal;
-  pkgJson?: PkgJson;
-  userExternals?: NonNullable<RsbuildConfig['output']>['externals'];
-}): RsbuildConfig => {
-  const { autoExternal, pkgJson, userExternals } = options;
-
-  if (!autoExternal) {
-    return {};
-  }
-
-  if (!pkgJson) {
-    logger.warn(
-      'autoExternal configuration will not be applied due to read package.json failed',
-    );
-    return {};
-  }
-
-  const externalOptions = {
-    dependencies: true,
-    peerDependencies: true,
-    devDependencies: false,
-    ...(autoExternal === true ? {} : autoExternal),
-  };
-
-  // User externals configuration has higher priority than autoExternal
-  // eg: autoExternal: ['react'], user: output: { externals: { react: 'react-1' } }
-  // Only handle the case where the externals type is object, string / string[] does not need to be processed, other types are too complex.
-  const userExternalKeys =
-    userExternals &&
-    Object.prototype.toString.call(userExternals) === '[object Object]'
-      ? Object.keys(userExternals)
-      : [];
-
-  const externals = (
-    ['dependencies', 'peerDependencies', 'devDependencies'] as const
-  )
-    .reduce<string[]>((prev, type) => {
-      if (externalOptions[type]) {
-        return pkgJson[type] ? prev.concat(Object.keys(pkgJson[type]!)) : prev;
-      }
-      return prev;
-    }, [])
-    .filter((name) => !userExternalKeys.includes(name));
-
-  return externals.length
-    ? {
-        output: {
-          externals,
-        },
-      }
-    : {};
-};
diff --git a/packages/core/tests/external.test.ts b/packages/core/tests/external.test.ts
index 6e194d33..95ab8288 100644
--- a/packages/core/tests/external.test.ts
+++ b/packages/core/tests/external.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it, vi } from 'vitest';
-import { composeAutoExternalConfig } from '../src/utils/external';
+import { composeAutoExternalConfig } from '../src/config';
 
 vi.mock('rslog');
 
@@ -28,6 +28,32 @@ describe('should composeAutoExternalConfig correctly', () => {
     });
   });
 
+  it('autoExternal will deduplication ', () => {
+    const result = composeAutoExternalConfig({
+      autoExternal: true,
+      pkgJson: {
+        dependencies: {
+          foo: '1.0.0',
+          foo1: '1.0.0',
+        },
+        devDependencies: {
+          bar: '1.0.0',
+        },
+        peerDependencies: {
+          baz: '1.0.0',
+          foo: '1.0.0',
+          foo1: '1.0.0',
+        },
+      },
+    });
+
+    expect(result).toEqual({
+      output: {
+        externals: ['foo', 'foo1', 'baz'],
+      },
+    });
+  });
+
   it('autoExternal is object', () => {
     const result = composeAutoExternalConfig({
       autoExternal: {