diff --git a/lib/websocket/extensions.rb b/lib/websocket/extensions.rb
index 14ca7ab..0b246d2 100644
--- a/lib/websocket/extensions.rb
+++ b/lib/websocket/extensions.rb
@@ -75,6 +75,7 @@ def generate_offer
     def activate(header)
       responses = Parser.parse_header(header)
       @sessions = []
+      activated = {}
 
       responses.each_offer do |name, params|
         unless record = @index[name]
@@ -83,6 +84,11 @@ def activate(header)
 
         ext, session = *record
 
+        if activated[name]
+          raise ExtensionError, %Q{Server sent multiple responses for extension "#{ name }"}
+        end
+        activated[name] = true
+
         if reserved = reserved?(ext)
           raise ExtensionError, %Q{Server sent two extension responses that use the RSV#{ reserved[0] }} +
                                 %Q{bit: "#{ reserved[1] }" and "#{ ext.name }"}
diff --git a/spec/websocket/extensions_spec.rb b/spec/websocket/extensions_spec.rb
index ccb305d..d566778 100644
--- a/spec/websocket/extensions_spec.rb
+++ b/spec/websocket/extensions_spec.rb
@@ -120,6 +120,13 @@
         expect { @extensions.activate("tar") }.not_to raise_error
       end
 
+      it "rejects duplicate responses for an extension without reserved bits" do
+        allow(@ext).to receive(:rsv1).and_return(false)
+        expect(@session).to receive(:activate).with({}).exactly(1).and_return(true)
+
+        expect { @extensions.activate("deflate, deflate") }.to raise_error(ExtensionError)
+      end
+
       it "raises if two extensions conflict on RSV bits" do
         expect { @extensions.activate("deflate, tar") }.to raise_error(ExtensionError)
       end