improve wording

Co-authored-by: Olaf Hartig <[email protected]>

Author: Tpt

spec/index.html

@@ -2684,7 +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>
+          <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>