mixins: don't duplicate rules with the same name (fixes less#49)

 - this maintains the behavior where all mixins that matched the call
   are applied
 - if the result of applying the mixins produces rules with the same
   name then the last rule with that name wins
 - an alternative approach would be to only apply the last mixin that
   matches the call

lib/less/tree/mixin.js

@@ -8,7 +8,7 @@ tree.mixin.Call = function (elements, args, index) {
 };
 tree.mixin.Call.prototype = {
     eval: function (env) {
-        var mixins, args, rules = [], match = false;
+        var mixins, args, rules = [], match = false, names = {};
 
         for (var i = 0; i < env.frames.length; i++) {
             if ((mixins = env.frames[i].find(this.selector)).length > 0) {
@@ -25,7 +25,14 @@ tree.mixin.Call.prototype = {
                     }
                 }
                 if (match) {
-                    return rules;
+                    // filter out rules with the same name.  let the last one win
+                    return rules.reduceRight(function (reduced, rule) {
+                        if (!names[rule.name]) {
+                            reduced.unshift(rule);
+                            names[rule.name] = 1;
+                        }
+                        return reduced;
+                    }, []);
                 } else {
                     throw { message: 'No matching definition was found for `' +
                                       this.selector.toCSS().trim() + '('      +

test/css/mixins-import.css

@@ -1,6 +1,15 @@
 .a {
-  mixed: 20;
+  foo: 'bar';
+  mixed: 25;
+  main: 'yes';
 }
 .b {
-  mixed: 20;
+  foo: 'bar';
+  mixed: 25;
+  main: 'yes';
+}
+#main {
+  foo: 'bar';
+  mixed: 25;
+  main: 'yes';
 }

test/less/import/mixins-import-vars.less

@@ -1,3 +1,4 @@
 .mixin() {
 	mixed: 20;
+	foo: 'bar';
 }

test/less/mixins-import.less

@@ -1,2 +1,11 @@
 @import "import/mixins-import-a";
-@import "import/mixins-import-b";
+@import "import/mixins-import-b";
+
+.mixin() {
+	mixed: 25;
+	main: 'yes';
+}
+
+#main {
+	.mixin
+}