Index: boost/parameter/aux_/arg_list.hpp
===================================================================
RCS file: /cvsroot/boost/boost/boost/parameter/aux_/arg_list.hpp,v
retrieving revision 1.10
diff -u -r1.10 arg_list.hpp
--- boost/parameter/aux_/arg_list.hpp	24 Oct 2005 17:41:23 -0000	1.10
+++ boost/parameter/aux_/arg_list.hpp	13 Nov 2005 00:28:18 -0000
@@ -13,6 +13,7 @@
 #include <boost/parameter/config.hpp>
 
 #include <boost/mpl/apply.hpp>
+#include <boost/mpl/assert.hpp>
 
 #include <boost/type_traits/add_reference.hpp>
 #include <boost/type_traits/is_same.hpp>
@@ -65,6 +66,10 @@
         };
     };
 
+    // Terminator for has_key, indicating that the keyword is unique
+    template <class Unique_keyword>
+    static char (& has_key(Unique_keyword const *) )[1];
+
 #if BOOST_WORKAROUND(BOOST_MSVC, <= 1300) \
     || (BOOST_WORKAROUND(__GNUC__, < 3)) \
     || BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x564))
@@ -182,6 +187,16 @@
         };
     };
 
+    // Overload for key_type, so the assert below will fire if the
+    // same keyword is used again
+    static char (& has_key(key_type const *) )[2];
+    using Next::has_key;
+
+    BOOST_MPL_ASSERT_MSG(
+        sizeof(Next::has_key(static_cast<key_type const *>(0))) == 1, 
+        duplicate_keyword, (key_type)
+    );
+
     //
     // Begin implementation of indexing operators for looking up
     // specific arguments by name