diff --git a/infection.json.dist b/infection.json.dist
index 605d106..13361a2 100644
--- a/infection.json.dist
+++ b/infection.json.dist
@@ -17,7 +17,9 @@
         "LogicalOr": false,
         "IfNegation": false,
         "InstanceOf_": false,
+        "UnwrapArrayMap": false,
         "ArrayItemRemoval": false,
+        "UnwrapArrayUnique": false,
         "MethodCallRemoval": false,
         "LogicalOrAllSubExprNegation": false,
         "LogicalOrSingleSubExprNegation": false
diff --git a/src/HttpHeaders.php b/src/HttpHeaders.php
index edf8b2f..4d19178 100644
--- a/src/HttpHeaders.php
+++ b/src/HttpHeaders.php
@@ -13,17 +13,25 @@ final class HttpHeaders
 {
     private array $values = [];
 
+    private function __construct()
+    {
+    }
+
+    public static function build(): HttpHeaders
+    {
+        return new HttpHeaders();
+    }
+
     public function add(Header $header): HttpHeaders
     {
-        $key = $header->key();
-        $this->values['header'][$key] = sprintf('%s: %s', $key, $header->value());
+        $this->values[$header->key()][] = $header->value();
 
         return $this;
     }
 
-    public function getHeader(): array
+    public function getHeader(string $key): array
     {
-        return $this->values['header'] ?? [];
+        return $this->values[$key] ?? [];
     }
 
     public function hasHeaders(): bool
@@ -33,11 +41,14 @@ public function hasHeaders(): bool
 
     public function hasHeader(string $key): bool
     {
-        return !empty($this->getHeader()[$key]);
+        return !empty($this->getHeader(key: $key));
     }
 
     public function toArray(): array
     {
-        return $this->values;
+        return array_map(
+            fn(array $values): array => [end($values)],
+            array_map(fn(array $values): array => array_unique($values), $this->values)
+        );
     }
 }
diff --git a/src/Internal/Response.php b/src/Internal/Response.php
index 999a4e0..0154b35 100644
--- a/src/Internal/Response.php
+++ b/src/Internal/Response.php
@@ -23,7 +23,7 @@ private function __construct(
     public static function from(HttpCode $code, mixed $data, ?HttpHeaders $headers): ResponseInterface
     {
         if (is_null($headers) || !$headers->hasHeaders()) {
-            $headers = (new HttpHeaders())->add(header: HttpContentType::APPLICATION_JSON);
+            $headers = HttpHeaders::build()->add(header: HttpContentType::APPLICATION_JSON);
         }
 
         return new Response(code: $code, body: StreamFactory::from(data: $data), headers: $headers);
@@ -76,12 +76,12 @@ public function hasHeader(string $name): bool
 
     public function getHeader(string $name): array
     {
-        return $this->headers->getHeader();
+        return $this->headers->getHeader(key: $name);
     }
 
     public function getHeaderLine(string $name): string
     {
-        return implode(', ', $this->getHeader(name: $name));
+        return implode(', ', $this->headers->getHeader(key: $name));
     }
 
     public function getBody(): StreamInterface
diff --git a/tests/HttpResponseTest.php b/tests/HttpResponseTest.php
index 28c0399..10e23a2 100644
--- a/tests/HttpResponseTest.php
+++ b/tests/HttpResponseTest.php
@@ -8,7 +8,12 @@
 
 class HttpResponseTest extends TestCase
 {
-    private array $defaultHeader = ['header' => ['Content-Type' => 'Content-Type: application/json']];
+    private array $defaultHeader;
+
+    protected function setUp(): void
+    {
+        $this->defaultHeader = ['Content-Type' => [HttpContentType::APPLICATION_JSON->value]];
+    }
 
     /**
      * @dataProvider providerData
diff --git a/tests/Internal/HeadersTest.php b/tests/Internal/HeadersTest.php
index 8dcfb82..dc34fba 100644
--- a/tests/Internal/HeadersTest.php
+++ b/tests/Internal/HeadersTest.php
@@ -10,19 +10,18 @@ class HeadersTest extends TestCase
 {
     public function testAddAndGetValues(): void
     {
-        $headers = (new HttpHeaders())->add(header: HttpContentType::APPLICATION_JSON);
-        $expected = ['header' => ['Content-Type' => 'Content-Type: application/json']];
+        $headers = HttpHeaders::build()->add(header: HttpContentType::APPLICATION_JSON);
+        $expected = ['Content-Type' => [HttpContentType::APPLICATION_JSON->value]];
 
         self::assertEquals($expected, $headers->toArray());
     }
 
     public function testAddAndGetUniqueValues(): void
     {
-        $headers = (new HttpHeaders())
+        $headers = HttpHeaders::build()
             ->add(header: HttpContentType::TEXT_HTML)
             ->add(header: HttpContentType::APPLICATION_PDF);
-
-        $expected = ['header' => ['Content-Type' => 'Content-Type: application/pdf']];
+        $expected = ['Content-Type' => [HttpContentType::APPLICATION_PDF->value]];
 
         self::assertEquals($expected, $headers->toArray());
     }
diff --git a/tests/Internal/ResponseTest.php b/tests/Internal/ResponseTest.php
index 1ae43cc..25d7f59 100644
--- a/tests/Internal/ResponseTest.php
+++ b/tests/Internal/ResponseTest.php
@@ -11,13 +11,10 @@
 
 class ResponseTest extends TestCase
 {
-    private const TEXT_PLAIN = 'Content-Type: text/plain';
-    private const APPLICATION_JSON = 'Content-Type: application/json';
-
     public function testDefaultHeaders(): void
     {
         $response = Response::from(code: HttpCode::OK, data: [], headers: null);
-        $expected = ['header' => ['Content-Type' => self::APPLICATION_JSON]];
+        $expected = ['Content-Type' => [HttpContentType::APPLICATION_JSON->value]];
 
         self::assertEquals($expected, $response->getHeaders());
     }
@@ -31,9 +28,9 @@ public function testGetProtocolVersion(): void
 
     public function testGetHeaders(): void
     {
-        $headers = (new HttpHeaders())->add(header: HttpContentType::APPLICATION_JSON);
+        $headers = HttpHeaders::build()->add(header: HttpContentType::APPLICATION_JSON);
         $response = Response::from(code: HttpCode::OK, data: [], headers: $headers);
-        $expected = ['Content-Type' => self::APPLICATION_JSON];
+        $expected = [HttpContentType::APPLICATION_JSON->value];
 
         self::assertEquals($headers->toArray(), $response->getHeaders());
         self::assertEquals($expected, $response->getHeader(name: 'Content-Type'));
@@ -41,9 +38,9 @@ public function testGetHeaders(): void
 
     public function testHasHeader(): void
     {
-        $headers = (new HttpHeaders())->add(header: HttpContentType::TEXT_PLAIN);
+        $headers = HttpHeaders::build()->add(header: HttpContentType::TEXT_PLAIN);
         $response = Response::from(code: HttpCode::OK, data: [], headers: $headers);
-        $expected = ['Content-Type' => self::TEXT_PLAIN];
+        $expected = [HttpContentType::TEXT_PLAIN->value];
 
         self::assertTrue($response->hasHeader(name: 'Content-Type'));
         self::assertEquals($expected, $response->getHeader(name: 'Content-Type'));
@@ -51,10 +48,10 @@ public function testHasHeader(): void
 
     public function testGetHeaderLine(): void
     {
-        $headers = (new HttpHeaders())->add(header: HttpContentType::APPLICATION_JSON);
+        $headers = HttpHeaders::build()->add(header: HttpContentType::APPLICATION_JSON);
         $response = Response::from(code: HttpCode::OK, data: [], headers: $headers);
 
-        self::assertEquals(self::APPLICATION_JSON, $response->getHeaderLine(name: 'Content-Type'));
+        self::assertEquals(HttpContentType::APPLICATION_JSON->value, $response->getHeaderLine(name: 'Content-Type'));
     }
 
     public function testExceptionWhenBadMethodCallOnWithBody(): void