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 }"}