Forbid repeated variables in VALUES

Close #293

Author: Tpt

spec/index.html

@@ -2684,6 +2684,7 @@ <h4>VALUES syntax</h4>
           <pre class="data nohighlight">
             VALUES (?z) { ("abc") ("def") }
           </pre>
+          <p>Note that duplicated variables are not allowed in VALUES clauses.</p>
         </section>
         <section id="inline-data-examples">
           <h4>VALUES Examples</h4>
@@ -11125,7 +11126,8 @@ <h3>Grammar</h3>
             nor in <code><a href="#rDeleteData">DELETE DATA</a></code>.</li>
           <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>
+            must be the same as the number of each list of associated values in the <code>DataBlock</code>,
+            and variables can not be repeated in the variable list.</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