Avoid running jQuery.unique() for methods that are guaranteed to produce a...

Avoid running jQuery.unique() for methods that are guaranteed to produce a unique result set. Fixes #7964 - Some traversal methods perform an unnecessary uniqueness check.

src/traversing.js

@@ -6,7 +6,14 @@ var runtil = /Until$/,
 	rmultiselector = /,/,
 	isSimple = /^.[^:#\[\.,]*$/,
 	slice = Array.prototype.slice,
-	POS = jQuery.expr.match.POS;
+	POS = jQuery.expr.match.POS,
+	// methods guaranteed to produce a unique set when starting from a unique set
+	guaranteedUnique = {
+		children: true,
+		contents: true,
+		next: true,
+		prev: true
+	};
 
 jQuery.fn.extend({
 	find: function( selector ) {
@@ -206,7 +213,7 @@ jQuery.each({
 			ret = jQuery.filter( selector, ret );
 		}
 
-		ret = this.length > 1 ? jQuery.unique( ret ) : ret;
+		ret = this.length > 1 && !guaranteedUnique[ name ] ? jQuery.unique( ret ) : ret;
 
 		if ( (this.length > 1 || rmultiselector.test( selector )) && rparentsprev.test( name ) ) {
 			ret = ret.reverse();