Don't append duplicate menu items

Fixes #995

spec/atom-spec.coffee

@@ -171,6 +171,7 @@ describe "the `atom` global", ->
 
               atom.activatePackage("package-with-menus")
 
+              expect(atom.menu.template.length).toBe 2
               expect(atom.menu.template[0].label).toBe "Second to Last"
               expect(atom.menu.template[1].label).toBe "Last"
               expect(atom.contextMenu.definitionsForElement(element)[0].label).toBe "Menu item 1"

spec/fixtures/packages/package-with-menus/menus/menu-3.cson

@@ -1,3 +1,7 @@
+'menu': [
+  { 'label': 'Second to Last' }
+]
+
 'context-menu':
   '.test-1':
     'Menu item 3': 'command-3'

spec/spec-helper.coffee

@@ -111,7 +111,6 @@ afterEach ->
   keymap.bindingSets = bindingSetsToRestore
   keymap.bindingSetsByFirstKeystroke = bindingSetsByFirstKeystrokeToRestore
   atom.deactivatePackages()
-  atom.menu.template = []
 
   window.rootView?.remove?()
   atom.rootView?.remove?() if atom.rootView isnt window.rootView

src/menu-manager.coffee

@@ -43,10 +43,11 @@ class MenuManager
   # appended to the bottom of existing menus where possible.
   merge: (menu, item) ->
     item = _.deepClone(item)
-    if item.submenu? and match = _.find(menu, (o) -> o.submenu? and o.label == item.label)
+
+    if item.submenu? and match = _.find(menu, (i) -> i.submenu? and i.label == item.label)
       @merge(match.submenu, i) for i in item.submenu
     else
-      menu.push(item)
+      menu.push(item) unless _.find(menu, (i) -> i.label == item.label)
 
   # Private
   sendToBrowserProcess: (template, keystrokesByCommand) ->