diff --git a/config.xsd b/config.xsd
index 2dc25540f..522f74a71 100644
--- a/config.xsd
+++ b/config.xsd
@@ -143,6 +143,7 @@
             <xs:element name="DocblockTypeContradiction" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="DuplicateArrayKey" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="DuplicateParam" type="IssueHandlerType" minOccurs="0" />
+            <xs:element name="DuplicateFunction" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="DuplicateClass" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="DuplicateMethod" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="EmptyArrayAccess" type="IssueHandlerType" minOccurs="0" />
diff --git a/docs/issues.md b/docs/issues.md
index 632e655a9..d62b3967f 100644
--- a/docs/issues.md
+++ b/docs/issues.md
@@ -180,6 +180,16 @@ class A {}
 class A {}
 ```
 
+### DuplicateFunction
+
+Emitted when a function is defined twice
+
+```php
+function foo() : void {}
+function bar() : void {}
+function foo() : void {}
+```
+
 ### DuplicateMethod
 
 Emitted when a method is defined twice
diff --git a/src/Psalm/Internal/Visitor/ReflectorVisitor.php b/src/Psalm/Internal/Visitor/ReflectorVisitor.php
index 6adbbfa1c..1a5bfaa8e 100644
--- a/src/Psalm/Internal/Visitor/ReflectorVisitor.php
+++ b/src/Psalm/Internal/Visitor/ReflectorVisitor.php
@@ -21,6 +21,7 @@ use Psalm\Exception\IncorrectDocblockException;
 use Psalm\Exception\TypeParseTreeException;
 use Psalm\FileSource;
 use Psalm\Issue\DuplicateClass;
+use Psalm\Issue\DuplicateFunction;
 use Psalm\Issue\DuplicateMethod;
 use Psalm\Issue\DuplicateParam;
 use Psalm\Issue\InvalidDocblock;
@@ -851,15 +852,46 @@ class ReflectorVisitor extends PhpParser\NodeVisitorAbstract implements PhpParse
                 ($this->aliases->namespace ? $this->aliases->namespace . '\\' : '') . $stmt->name->name;
             $function_id = strtolower($cased_function_id);
 
-            if (isset($this->file_storage->functions[$function_id])) {
-                if ($this->codebase->register_stub_files || $this->codebase->register_autoload_files) {
+            if ($this->codebase->register_stub_files || $this->codebase->register_autoload_files) {
+                if (isset($this->file_storage->functions[$function_id])) {
                     $this->codebase->functions->addGlobalFunction(
                         $function_id,
                         $this->file_storage->functions[$function_id]
                     );
-                }
 
-                return $this->file_storage->functions[$function_id];
+                    return $this->file_storage->functions[$function_id];
+                }
+            } else {
+                if (isset($this->file_storage->functions[$function_id])) {
+                    $duplicate_function_storage = $this->file_storage->functions[$function_id];
+
+                    if (IssueBuffer::accepts(
+                        new DuplicateFunction(
+                            'Method ' . $function_id . ' has already been defined'
+                                . ($duplicate_function_storage->location
+                                    ? ' in ' . $duplicate_function_storage->location->file_path
+                                    : ''),
+                            new CodeLocation($this->file_scanner, $stmt, null, true)
+                        )
+                    )) {
+                        // fall through
+                    }
+
+                    $this->file_storage->has_visitor_issues = true;
+
+                    $duplicate_function_storage->has_visitor_issues = true;
+
+                    return $this->file_storage->functions[$function_id];
+                } elseif (isset($this->config->getPredefinedFunctions()[$function_id])) {
+                    if (IssueBuffer::accepts(
+                        new DuplicateFunction(
+                            'Method ' . $function_id . ' has already been defined as a core function',
+                            new CodeLocation($this->file_scanner, $stmt, null, true)
+                        )
+                    )) {
+                        // fall through
+                    }
+                }
             }
 
             $storage = new FunctionLikeStorage();
diff --git a/src/Psalm/Issue/DuplicateFunction.php b/src/Psalm/Issue/DuplicateFunction.php
new file mode 100644
index 000000000..b9543980e
--- /dev/null
+++ b/src/Psalm/Issue/DuplicateFunction.php
@@ -0,0 +1,6 @@
+<?php
+namespace Psalm\Issue;
+
+class DuplicateFunction extends CodeIssue
+{
+}
diff --git a/tests/FunctionCallTest.php b/tests/FunctionCallTest.php
index eb3ec9715..caa81f793 100644
--- a/tests/FunctionCallTest.php
+++ b/tests/FunctionCallTest.php
@@ -1328,6 +1328,12 @@ class FunctionCallTest extends TestCase
                         foo($x);
                     }',
             ],
+            'duplicateNamespacedFunction' => [
+                '<?php
+                    namespace Bar;
+
+                    function sort() : void {}',
+            ],
         ];
     }
 
@@ -1728,6 +1734,17 @@ class FunctionCallTest extends TestCase
                     }',
                 'error_message' => 'InvalidScalarArgument',
             ],
+            'duplicateFunction' => [
+                '<?php
+                    function f() : void {}
+                    function f() : void {}',
+                'error_message' => 'DuplicateFunction',
+            ],
+            'duplicateCoreFunction' => [
+                '<?php
+                    function sort() : void {}',
+                'error_message' => 'DuplicateFunction',
+            ],
         ];
     }
 }
diff --git a/tests/LanguageServer/SymbolLookupTest.php b/tests/LanguageServer/SymbolLookupTest.php
index d04741f85..9ff610473 100644
--- a/tests/LanguageServer/SymbolLookupTest.php
+++ b/tests/LanguageServer/SymbolLookupTest.php
@@ -77,7 +77,6 @@ class SymbolLookupTest extends \Psalm\Tests\TestCase
 
         $codebase = $this->project_analyzer->getCodebase();
 
-        $codebase->scanFiles();
         $this->analyzeFile('somefile.php', new Context());
 
         $this->assertSame('<?php public function foo() : void', $codebase->getSymbolInformation('somefile.php', 'B\A::foo()'));
@@ -114,7 +113,6 @@ class SymbolLookupTest extends \Psalm\Tests\TestCase
 
         $codebase = $this->project_analyzer->getCodebase();
 
-        $codebase->scanFiles();
         $this->analyzeFile('somefile.php', new Context());
 
 
diff --git a/tests/Php56Test.php b/tests/Php56Test.php
index 8be124c7c..2287c4f73 100644
--- a/tests/Php56Test.php
+++ b/tests/Php56Test.php
@@ -236,7 +236,7 @@ class Php56Test extends TestCase
                         return intval(...$args);
                     }
 
-                    function foo(ArrayIterator $args): int {
+                    function bar(ArrayIterator $args): int {
                         return intval(...$args);
                     }',
             ],
diff --git a/tests/TypeReconciliationTest.php b/tests/TypeReconciliationTest.php
index ea48dd7e3..94f54d848 100644
--- a/tests/TypeReconciliationTest.php
+++ b/tests/TypeReconciliationTest.php
@@ -872,7 +872,7 @@ class TypeReconciliationTest extends TestCase
                         if ($i == 0.0) {}
                         if (0.0 == $i) {}
                     }
-                    function foo(float $i) : void {
+                    function bar(float $i) : void {
                         $i = $i / 100.0;
                         if ($i == "5") {}
                         if ("5" == $i) {}
@@ -883,7 +883,7 @@ class TypeReconciliationTest extends TestCase
                         if ($i == 0) {}
                         if (0 == $i) {}
                     }
-                    function foo(string $i) : void {
+                    function bat(string $i) : void {
                         if ($i == 5) {}
                         if (5 == $i) {}
                         if ($i == 5.0) {}