diff --git a/lib/Plack/Middleware/Zstandard.pm b/lib/Plack/Middleware/Zstandard.pm
index c1a81bb..795e977 100644
--- a/lib/Plack/Middleware/Zstandard.pm
+++ b/lib/Plack/Middleware/Zstandard.pm
@@ -29,6 +29,9 @@ package Plack::Middleware::Zstandard {
       push @vary, 'Accept-Encoding';
       $h->set('Vary' => join(",", @vary));
 
+      # Do not clobber already existing encoding
+      return if $h->exists('Content-Encoding') && $h->get('Content-Encoding') ne 'identity';
+
       return undef unless ($env->{HTTP_ACCEPT_ENCODING} // '') =~ /\bzstd\b/;
 
       $h->set('Content-Encoding' => 'zstd');
diff --git a/t/plack_middleware_zstandard.t b/t/plack_middleware_zstandard.t
index 78f4622..21ed551 100644
--- a/t/plack_middleware_zstandard.t
+++ b/t/plack_middleware_zstandard.t
@@ -107,6 +107,32 @@ subtest 'basic' => sub {
 
   };
 
+  subtest 'do not clobber' => sub {
+
+    my $content = 'Hello World';
+    local @res = (
+      200, [
+        'Content-Type' => 'text/plain',
+        'Content-ENcoding' => 'foo',
+      ], [
+        $content
+      ]
+    );
+
+    req(
+      GET('/', 'Accept-Encoding' => 'zstd'),
+      res {
+        code 200;
+        header 'Content-Encoding' => 'foo';
+        header 'Vary', 'Accept-Encoding';
+        call content => 'Hello World';
+      },
+    );
+
+    note_debug();
+
+  };
+
   subtest 'No accept' => sub {
 
     my $content = 'Hello World';