diff --git a/build.xml b/build.xml
index 3fd9b25d4a..6a163cbbdd 100644
--- a/build.xml
+++ b/build.xml
@@ -134,6 +134,8 @@
 			<arg value="--exclude"/>
 			<arg path="tests/PHPStan/Rules/Arrays/data/offset-access-without-dim-for-reading.php"/>
 			<arg value="--exclude"/>
+			<arg path="tests/PHPStan/Rules/Classes/data/duplicate-declarations.php"/>
+			<arg value="--exclude"/>
 			<arg path="tests/PHPStan/Rules/Classes/data/extends-error.php"/>
 			<arg value="--exclude"/>
 			<arg path="tests/PHPStan/Rules/Classes/data/implements-error.php"/>
diff --git a/conf/config.level0.neon b/conf/config.level0.neon
index a9ec38d8af..e7396651e3 100644
--- a/conf/config.level0.neon
+++ b/conf/config.level0.neon
@@ -16,6 +16,7 @@ rules:
 	- PHPStan\Rules\Arrays\OffsetAccessWithoutDimForReadingRule
 	- PHPStan\Rules\Classes\ClassConstantDeclarationRule
 	- PHPStan\Rules\Classes\ClassConstantRule
+	- PHPStan\Rules\Classes\DuplicateDeclarationRule
 	- PHPStan\Rules\Classes\ExistingClassesInClassImplementsRule
 	- PHPStan\Rules\Classes\ExistingClassesInInterfaceExtendsRule
 	- PHPStan\Rules\Classes\ExistingClassInClassExtendsRule
diff --git a/src/Rules/Classes/DuplicateDeclarationRule.php b/src/Rules/Classes/DuplicateDeclarationRule.php
new file mode 100644
index 0000000000..3e84f48fdf
--- /dev/null
+++ b/src/Rules/Classes/DuplicateDeclarationRule.php
@@ -0,0 +1,79 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Classes;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Node\InClassNode;
+use PHPStan\Rules\RuleErrorBuilder;
+use function array_key_exists;
+use function sprintf;
+use function strtolower;
+
+/**
+ * @implements \PHPStan\Rules\Rule<\PHPStan\Node\InClassNode>
+ */
+class DuplicateDeclarationRule implements \PHPStan\Rules\Rule
+{
+
+	public function getNodeType(): string
+	{
+		return InClassNode::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		$classReflection = $scope->getClassReflection();
+		if ($classReflection === null) {
+			throw new \PHPStan\ShouldNotHappenException();
+		}
+
+		$errors = [];
+
+		$declaredClassConstants = [];
+		foreach ($node->getOriginalNode()->getConstants() as $constDecl) {
+			foreach ($constDecl->consts as $const) {
+				if (array_key_exists($const->name->name, $declaredClassConstants)) {
+					$errors[] = RuleErrorBuilder::message(sprintf(
+						'Cannot redeclare constant %s::%s.',
+						$classReflection->getDisplayName(),
+						$const->name->name
+					))->line($const->getLine())->nonIgnorable()->build();
+				} else {
+					$declaredClassConstants[$const->name->name] = true;
+				}
+			}
+		}
+
+		$declaredProperties = [];
+		foreach ($node->getOriginalNode()->getProperties() as $propertyDecl) {
+			foreach ($propertyDecl->props as $property) {
+				if (array_key_exists($property->name->name, $declaredProperties)) {
+					$errors[] = RuleErrorBuilder::message(sprintf(
+						'Cannot redeclare property %s::$%s.',
+						$classReflection->getDisplayName(),
+						$property->name->name
+					))->line($property->getLine())->nonIgnorable()->build();
+				} else {
+					$declaredProperties[$property->name->name] = true;
+				}
+			}
+		}
+
+		$declaredFunctions = [];
+		foreach ($node->getOriginalNode()->getMethods() as $method) {
+			if (array_key_exists(strtolower($method->name->name), $declaredFunctions)) {
+				$errors[] = RuleErrorBuilder::message(sprintf(
+					'Cannot redeclare method %s::%s().',
+					$classReflection->getDisplayName(),
+					$method->name->name
+				))->line($method->getStartLine())->nonIgnorable()->build();
+			} else {
+				$declaredFunctions[strtolower($method->name->name)] = true;
+			}
+		}
+
+		return $errors;
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Classes/DuplicateDeclarationRuleTest.php b/tests/PHPStan/Rules/Classes/DuplicateDeclarationRuleTest.php
new file mode 100644
index 0000000000..85b0f532ec
--- /dev/null
+++ b/tests/PHPStan/Rules/Classes/DuplicateDeclarationRuleTest.php
@@ -0,0 +1,58 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Classes;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends \PHPStan\Testing\RuleTestCase<\PHPStan\Rules\Classes\DuplicateDeclarationRule>
+ */
+class DuplicateDeclarationRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new DuplicateDeclarationRule();
+	}
+
+	public function testDuplicateDeclarations(): void
+	{
+		if (!self::$useStaticReflectionProvider) {
+			$this->markTestSkipped('This test needs static reflection');
+		}
+
+		$this->analyse(
+			[
+				__DIR__ . '/data/duplicate-declarations.php',
+			],
+			[
+				[
+					'Cannot redeclare constant DuplicateDeclarations\Foo::CONST1.',
+					8,
+				],
+				[
+					'Cannot redeclare constant DuplicateDeclarations\Foo::CONST2.',
+					10,
+				],
+				[
+					'Cannot redeclare property DuplicateDeclarations\Foo::$prop1.',
+					17,
+				],
+				[
+					'Cannot redeclare property DuplicateDeclarations\Foo::$prop2.',
+					20,
+				],
+				[
+					'Cannot redeclare method DuplicateDeclarations\Foo::func1().',
+					27,
+				],
+				[
+					'Cannot redeclare method DuplicateDeclarations\Foo::Func1().',
+					35,
+				],
+			]
+		);
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Classes/data/duplicate-declarations.php b/tests/PHPStan/Rules/Classes/data/duplicate-declarations.php
new file mode 100644
index 0000000000..7deff4c5e3
--- /dev/null
+++ b/tests/PHPStan/Rules/Classes/data/duplicate-declarations.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace DuplicateDeclarations;
+
+class Foo{
+
+	public const CONST1 = 1;
+	public const CONST1 = 2;
+
+	public const CONST2 = 2, CONST2 = 1;
+
+	public const CONST3 = 1;
+
+	/** @var int */
+	public $prop1;
+	/** @var int */
+	public $prop1;
+
+	/** @var int */
+	public $prop2, $prop2;
+
+	/** @var int */
+	public $prop3;
+
+	public function func1() : void{}
+
+	public function func1() : int{
+		return 1;
+	}
+
+	public function func2() : int{
+		return 2;
+	}
+
+	public function Func1() : void{}
+}