diff --git a/Lib/idlelib/IOBinding.py b/Lib/idlelib/IOBinding.py
--- a/Lib/idlelib/IOBinding.py
+++ b/Lib/idlelib/IOBinding.py
@@ -10,6 +10,7 @@
 from tkinter.simpledialog import askstring
 
 from idlelib.configHandler import idleConf
+from idlelib import macosxSupport
 
 from codecs import BOM_UTF8
 
@@ -108,7 +109,13 @@
         self.text = editwin.text
         self.__id_open = self.text.bind("<<open-window-from-file>>", self.open)
         self.__id_save = self.text.bind("<<save-window>>", self.save)
-        self.__id_saveas = self.text.bind("<<save-window-as-file>>",
+        if macosxSupport.runningAsOSXApp() and not macosxSupport.isCarbonAquaTk():
+            # Tk-Cocoa on MacOSX causes a second window to be opened when
+            # we explicitly add a keybinding. Why this is so is not yet known,
+            # this test is therefore a hack.
+            self.__id_saveas = None
+        else:
+            self.__id_saveas = self.text.bind("<<save-window-as-file>>",
                                           self.save_as)
         self.__id_savecopy = self.text.bind("<<save-copy-of-window-as-file>>",
                                             self.save_a_copy)
@@ -119,7 +126,8 @@
         # Undo command bindings
         self.text.unbind("<<open-window-from-file>>", self.__id_open)
         self.text.unbind("<<save-window>>", self.__id_save)
-        self.text.unbind("<<save-window-as-file>>",self.__id_saveas)
+        if self.__id_saveas is not None:
+            self.text.unbind("<<save-window-as-file>>",self.__id_saveas)
         self.text.unbind("<<save-copy-of-window-as-file>>", self.__id_savecopy)
         self.text.unbind("<<print-window>>", self.__id_print)
         # Break cycles