diff --git a/src/Composer/Autoload/AutoloadGenerator.php b/src/Composer/Autoload/AutoloadGenerator.php
index be776f1d0..3e2faa3eb 100644
--- a/src/Composer/Autoload/AutoloadGenerator.php
+++ b/src/Composer/Autoload/AutoloadGenerator.php
@@ -679,10 +679,26 @@ EOF;
      */
     protected function getIncludeFilesFile(array $files, Filesystem $filesystem, string $basePath, string $vendorPath, string $vendorPathCode, string $appBaseDirCode)
     {
+        // Get the path to each file, and make sure these paths are unique.
+        $files = array_map(
+            function (string $functionFile) use ($filesystem, $basePath, $vendorPath): string {
+                return $this->getPathCode($filesystem, $basePath, $vendorPath, $functionFile);
+            },
+            $files
+        );
+        $uniqueFiles = array_unique($files);
+        if (count($uniqueFiles) < count($files)) {
+            $this->io->writeError('<warning>The following "files" autoload rules are included multiple times, this may cause issues and should be resolved:</warning>');
+            foreach (array_unique(array_diff_assoc($files, $uniqueFiles)) as $duplicateFile) {
+                $this->io->writeError('<warning> - '.$duplicateFile.'</warning>');
+            }
+        }
+        unset($uniqueFiles);
+
         $filesCode = '';
+
         foreach ($files as $fileIdentifier => $functionFile) {
-            $filesCode .= '    ' . var_export($fileIdentifier, true) . ' => '
-                . $this->getPathCode($filesystem, $basePath, $vendorPath, $functionFile) . ",\n";
+            $filesCode .= '    ' . var_export($fileIdentifier, true) . ' => ' . $functionFile . ",\n";
         }
 
         if (!$filesCode) {
diff --git a/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php b/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
index fda3b06ed..cc8c2f0ac 100644
--- a/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
+++ b/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
@@ -14,6 +14,8 @@ namespace Composer\Test\Autoload;
 
 use Composer\Autoload\AutoloadGenerator;
 use Composer\Filter\PlatformRequirementFilter\PlatformRequirementFilterFactory;
+use Composer\IO\BufferIO;
+use Composer\IO\IOInterface;
 use Composer\Package\CompletePackage;
 use Composer\Package\Link;
 use Composer\Package\Version\VersionParser;
@@ -74,6 +76,11 @@ class AutoloadGeneratorTest extends TestCase
      */
     private $fs;
 
+    /**
+     * @var BufferIO
+     */
+    private $io;
+
     /**
      * @var EventDispatcher&MockObject
      */
@@ -109,6 +116,8 @@ class AutoloadGeneratorTest extends TestCase
             },
         ];
 
+        $this->io = new BufferIO();
+
         $this->config->expects($this->atLeastOnce())
             ->method('get')
             ->will($this->returnCallback(function ($arg) {
@@ -149,7 +158,7 @@ class AutoloadGeneratorTest extends TestCase
             ->disableOriginalConstructor()
             ->getMock();
 
-        $this->generator = new AutoloadGenerator($this->eventDispatcher);
+        $this->generator = new AutoloadGenerator($this->eventDispatcher, $this->io);
     }
 
     protected function tearDown(): void
@@ -373,6 +382,31 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('classmap6', $this->vendorDir.'/composer', 'classmap');
     }
 
+    public function testDuplicateFilesWarning(): void
+    {
+        $package = new RootPackage('root/a', '1.0', '1.0');
+        $package->setAutoload([
+            'files' => ['foo.php', 'bar.php', './foo.php', '././foo.php'],
+        ]);
+
+        $this->repository->expects($this->once())
+            ->method('getCanonicalPackages')
+            ->will($this->returnValue([]));
+
+        $this->fs->ensureDirectoryExists($this->vendorDir.'/a');
+        $this->fs->ensureDirectoryExists($this->workingDir.'/src');
+        $this->fs->ensureDirectoryExists($this->workingDir.'/lib');
+
+        file_put_contents($this->workingDir.'/foo.php', '<?php class FilesFoo {}');
+        file_put_contents($this->workingDir.'/bar.php', '<?php class FilesBar {}');
+
+        $this->generator->dump($this->config, $this->repository, $package, $this->im, 'composer', false, 'FilesWarning');
+        self::assertFileContentEquals(__DIR__.'/Fixtures/autoload_files_duplicates.php', $this->vendorDir.'/composer/autoload_files.php');
+        $expected = '<warning>The following "files" autoload rules are included multiple times, this may cause issues and should be resolved:</warning>'.PHP_EOL.
+            '<warning> - $baseDir . \'/foo.php\'</warning>'.PHP_EOL;
+        self::assertEquals($expected, $this->io->getOutput());;
+    }
+
     public function testVendorsAutoloading(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
diff --git a/tests/Composer/Test/Autoload/Fixtures/autoload_files_duplicates.php b/tests/Composer/Test/Autoload/Fixtures/autoload_files_duplicates.php
new file mode 100644
index 000000000..d1c5e0841
--- /dev/null
+++ b/tests/Composer/Test/Autoload/Fixtures/autoload_files_duplicates.php
@@ -0,0 +1,13 @@
+<?php
+
+// autoload_files.php @generated by Composer
+
+$vendorDir = dirname(__DIR__);
+$baseDir = dirname($vendorDir);
+
+return array(
+    'b4b4f2eb2151c57cadbd5714a7aba8b7' => $baseDir . '/foo.php',
+    '99b24fc198db06c1d2d8118a8a5475eb' => $baseDir . '/bar.php',
+    '6bad5af0771cca3d076e69b25d0791bb' => $baseDir . '/foo.php',
+    '51841489e2c601aedd3623cb72708483' => $baseDir . '/foo.php',
+);