diff --git a/.gitignore b/.gitignore
index 7cae452b..a5a487f7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,4 +8,5 @@
 /vendor-bin/**/vendor
 /.php_cs.cache
 /.phpunit-cache/
+/.phpunit.cache/
 /.dump.rdb
diff --git a/src/SurrogateKeys.php b/src/SurrogateKeys.php
index 073120a2..f9ff9a42 100644
--- a/src/SurrogateKeys.php
+++ b/src/SurrogateKeys.php
@@ -38,14 +38,11 @@ public function addTag(ResourceObject $ro): void
 
     public function setSurrogateHeader(ResourceObject $ro): void
     {
-        $key = implode(' ', array_unique($this->surrogateKeys));
-        $wasSetManually = isset($ro->headers[Header::SURROGATE_KEY]);
-        if ($wasSetManually) {
-            $ro->headers[Header::SURROGATE_KEY] .= ' ' . $key;
-
-            return;
+        $keys = $this->surrogateKeys;
+        if (isset($ro->headers[Header::SURROGATE_KEY])) {
+            $keys = array_merge(explode(' ', $ro->headers[Header::SURROGATE_KEY]), $keys);
         }
 
-        $ro->headers[Header::SURROGATE_KEY] = $key;
+        $ro->headers[Header::SURROGATE_KEY] = implode(' ', array_unique($keys));
     }
 }
diff --git a/tests/SurrogateKeysTest.php b/tests/SurrogateKeysTest.php
index 7206373d..3ca95bda 100644
--- a/tests/SurrogateKeysTest.php
+++ b/tests/SurrogateKeysTest.php
@@ -73,4 +73,23 @@ public function testOnePurgeKey(): void
         $etags->setSurrogateHeader($foo);
         $this->assertSame('_foo_', $foo->headers[Header::SURROGATE_KEY]);
     }
+
+    public function testSetSurrogateHeaderDeduplicatesWithManualKeys(): void
+    {
+        $uri = new Uri('app://self/foo');
+        $etags = new SurrogateKeys($uri);
+        $child = new class extends ResourceObject{
+            /** @var array<string, string> */
+            public $headers = [Header::SURROGATE_KEY => 'shared-tag']; // phpcs:ignore
+        };
+        $child->uri = new Uri('app://self/foo1');
+        $etags->addTag($child);
+        $ro = new class extends ResourceObject{
+            /** @var array<string, string> */
+            public $headers = [Header::SURROGATE_KEY => 'shared-tag _foo_']; // phpcs:ignore
+        };
+        $ro->uri = $uri;
+        $etags->setSurrogateHeader($ro);
+        $this->assertSame('shared-tag _foo_ _foo1_', $ro->headers[Header::SURROGATE_KEY]);
+    }
 }