Forbid duplicated variables in VALUES

spec/index.html

@@ -2684,6 +2684,7 @@ <h4>VALUES syntax</h4>
           <pre class="data nohighlight">
             VALUES (?z) { ("abc") ("def") }
           </pre>
+          <p>Note that the same variable cannot be mentioned multiple times within the variables list of a VALUES clause.</p>
         </section>
         <section id="inline-data-examples">
           <h4>VALUES Examples</h4>
@@ -11126,6 +11127,7 @@ <h3>Grammar</h3>
           <li>Rules for limiting the use of blank node identifiers are given in <a href="#grammarBNodes">section 19.6</a>.</li>
           <li>The number of variables in the variable list of <code>VALUES</code> block 
             must be the same as the number of each list of associated values in the <code>DataBlock</code>.</li>
+          <li>Variables in the variable list of <code>VALUES</code> block must be unique.</li>
           <li>Variables introduced by <code>AS</code> in a <code>SELECT</code> clause
             must not already be <a href="#variableScope">in-scope</a>.</li>
           <li>The variable assigned in a <code>BIND</code> clause must not be already 
@@ -12614,6 +12616,7 @@ <h2>Changes between SPARQL 1.1 Query Language and SPARQL 1.2 Query Language</h2>
                 <li>Migrate XML Schema references to 1.1</li>
                 <li>Update references to XPath from 2.0 to 3.1</li>
                 <li>Define `EBV` as a functional form.</li>
+                <li>Forbid duplicated variables in `VALUES`.</li>
             </ul>
         </li>
         <li>