Do not load both preload files when both conditions hold

bin/rector.php

@@ -116,10 +116,8 @@ public function loadIfExistsAndNotLoadedYet(string $filePath): void
 
 if (file_exists(__DIR__ . '/../preload.php') && is_dir(__DIR__ . '/../vendor')) {
     require_once __DIR__ . '/../preload.php';
-}
-
-// require rector-src on split packages
-if (file_exists(__DIR__ . '/../preload-split-package.php') && is_dir(__DIR__ . '/../../../../vendor')) {
+} elseif (file_exists(__DIR__ . '/../preload-split-package.php') && is_dir(__DIR__ . '/../../../../vendor')) {
+    // rector-src on split packages
     require_once __DIR__ . '/../preload-split-package.php';
 }
 

tests/Bin/PreloadTest.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Rector\Tests\Bin;
+
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Process\Process;
+
+final class PreloadTest extends TestCase
+{
+    private string $rootDirectory;
+
+    protected function tearDown(): void
+    {
+        if (! isset($this->rootDirectory)) {
+            return;
+        }
+
+        $process = Process::fromShellCommandline('rm -rf ' . escapeshellarg($this->rootDirectory));
+        $process->run();
+    }
+
+    /**
+     * bin/rector.php picks a preload file from two conditions: "I have my own vendor/", so a
+     * monorepo checkout, and "I sit inside a project's vendor/". A checkout with its own vendor/
+     * that also happens to be three levels below another vendor/ answers yes to both. Both preload
+     * files declare isPHPStanTestPreloaded(), so loading both is a fatal error.
+     */
+    public function testPicksOnePreloadFileWhenBothConditionsHold(): void
+    {
+        $this->rootDirectory = sys_get_temp_dir() . '/rector-preload-test-' . uniqid();
+        $repositoryDirectory = $this->rootDirectory . '/a/b/c/repo';
+        mkdir($repositoryDirectory . '/bin', 0777, true);
+
+        $vendorDirectory = realpath(__DIR__ . '/../../vendor');
+        // the first condition, and the paths preload.php requires
+        symlink($vendorDirectory, $repositoryDirectory . '/vendor');
+        // the second condition, and the paths preload-split-package.php requires
+        symlink($vendorDirectory, $this->rootDirectory . '/a/vendor');
+
+        foreach (['bin/rector.php', 'preload.php', 'preload-split-package.php'] as $relativeFilePath) {
+            copy(__DIR__ . '/../../' . $relativeFilePath, $repositoryDirectory . '/' . $relativeFilePath);
+        }
+
+        $process = Process::fromShellCommandline(PHP_BINARY . ' bin/rector.php --version', $repositoryDirectory);
+        $process->run();
+
+        $this->assertStringNotContainsString('Cannot redeclare', $process->getErrorOutput());
+    }
+}