diff --git a/src/Configuration.php b/src/Configuration.php
index c1b4874..b08d7f4 100644
--- a/src/Configuration.php
+++ b/src/Configuration.php
@@ -65,11 +65,13 @@ public function __construct(array $baseSchemas = [])
      *
      * @psalm-allow-private-mutation
      */
-    public function addSchema(string $key, Schema $schema): void
+    public function addSchema(string $key, Schema $schema, bool $overwrite = true): void
     {
         $this->invalidate();
 
-        $this->configSchemas[$key] = $schema;
+        if ($overwrite || !isset($this->configSchemas[$key])) {
+            $this->configSchemas[$key] = $schema;
+        }
     }
 
     /**
diff --git a/tests/ConfigurationTest.php b/tests/ConfigurationTest.php
index 08851d5..f2ff4b1 100644
--- a/tests/ConfigurationTest.php
+++ b/tests/ConfigurationTest.php
@@ -21,6 +21,30 @@
 
 final class ConfigurationTest extends TestCase
 {
+    public function testNotOverrideSchema(): void
+    {
+        $config = new Configuration([
+            'a' => Expect::string()->default('foo')
+        ]);
+
+        // the previous schema will not be modified
+        $config->addSchema('a', Expect::int()->default(4), false);
+
+        $this->assertSame('foo', $config->get('a'));
+    }
+
+    public function testOverrideSchema(): void
+    {
+        $config = new Configuration([
+            'a' => Expect::string()->default('foo')
+        ]);
+
+        // the previous schema will be modified
+        $config->addSchema('a', Expect::int()->default(42));
+
+        $this->assertSame(42, $config->get('a'));
+    }
+
     public function testAddSchema(): void
     {
         $config = new Configuration();