Brought back the compareDocumentPosition code. While it is redundant in Firefox it's much faster than using indexOf.

Author: jeresig

src/selector.js

@@ -684,7 +684,15 @@ try {
 
 var sortOrder;
 
-if ( Array.prototype.indexOf ) {
+if ( document.documentElement.compareDocumentPosition ) {
+	sortOrder = function( a, b ) {
+		var ret = a.compareDocumentPosition(b) & 4 ? -1 : a === b ? 0 : 1;
+		if ( ret === 0 ) {
+			hasDuplicate = true;
+		}
+		return ret;
+	};
+} else if ( Array.prototype.indexOf ) {
 	var indexOf = Array.prototype.indexOf,
 		allSort = document.getElementsByTagName("*");
 
@@ -695,7 +703,7 @@ if ( Array.prototype.indexOf ) {
 		}
 		return ret;
 	};
-} else if ( document.documentElement.sourceIndex === 1 ) {
+} else if ( "sourceIndex" in document.documentElement ) {
 	sortOrder = function( a, b ) {
 		var ret = a.sourceIndex - b.sourceIndex;
 		if ( ret === 0 ) {

test/unit/selector.js

@@ -1,7 +1,7 @@
 module("selector");
 
 test("element", function() {
-	expect(17);
+	expect(18);
 	reset();
 
 	ok( jQuery("*").size() >= 30, "Select all" );
@@ -25,7 +25,7 @@ test("element", function() {
 	ok( jQuery("#lengthtest input").length, '<input name="length"> cannot be found under IE, see #945' );
 
 	// Check for unique-ness and sort order
-	//isSet( jQuery("*"), jQuery("*, *"), "Check for duplicates: *, *" );
+	isSet( jQuery("*"), jQuery("*, *"), "Check for duplicates: *, *" );
 	isSet( jQuery("p"), jQuery("p, div p"), "Check for duplicates: p, div p" );
 
 	t( "Checking sort order", "h2, h1", ["header", "banner", "userAgent"] );