diff --git a/phpunit/src/DuplicateTest.php b/phpunit/src/DuplicateTest.php
index 767ef6ee..5653c11c 100644
--- a/phpunit/src/DuplicateTest.php
+++ b/phpunit/src/DuplicateTest.php
@@ -13,6 +13,7 @@ class DuplicateTest extends TestCase
             $compiler = CompilerTest::create(ROOT_PATH);
             $testFile = __DIR__ . '/../code/' . $file;
             $compiler->addFiles([$testFile]);
+            $compiler->prepare($testFile);
             $compiler->convert($testFile);
         } catch (TestError $exception) {
             $this->assertStringContainsString($expected, $exception->getMessage());
diff --git a/src/Php/Preprocessor.php b/src/Php/Preprocessor.php
index b0221e11..3689c70a 100644
--- a/src/Php/Preprocessor.php
+++ b/src/Php/Preprocessor.php
@@ -197,7 +197,11 @@ class Preprocessor extends CompilerBase
         if ($this->stubFile) {
             $this->addFunction($name, $this->parseFunctionDecl($v));
         } else {
-            $this->symbolDeclInFile[strtolower($name)] = $this->file;
+            $functionNameLower = strtolower($name);
+            if (isset($this->symbolDeclInFile[$functionNameLower])) {
+                $this->fatalError($v, "Duplicate function `{$functionNameLower}`");
+            }
+            $this->symbolDeclInFile[$functionNameLower] = $this->file;
         }
     }
 
@@ -227,6 +231,11 @@ class Preprocessor extends CompilerBase
                 $this->symbolCallInFile[$this->file][] = $parentClassLower;
             }
         }
+
+        if (isset($this->symbolDeclInFile[$fullClassNameLower])) {
+            $this->fatalError($class, "Duplicate class `{$fullClassName}`");
+        }
+
         $this->symbolDeclInFile[$fullClassNameLower] = $this->file;
 
         $code = '';