Improve wording

Co-authored-by: Andy Seaborne <[email protected]>

Author: Tpt

spec/index.html

@@ -11126,8 +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>,
-            and variables can not be repeated in the variable list.</li>
+            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