bpo-21519: IDLE basic custom key entry better detects duplicates. (#2428)

diff --git a/Lib/idlelib/config_key.py b/Lib/idlelib/config_key.py
index 8d575ec..fc6e341 100644
--- a/Lib/idlelib/config_key.py
+++ b/Lib/idlelib/config_key.py
@@ -250,10 +250,10 @@
         '''
         finalKey = self.listKeysFinal.get(ANCHOR)
         modifiers = self.GetModifiers()
-        # create a key sequence list for overlap check:
-        keySequence = keys.split()
         keysOK = False
         title = self.keyerror_title
+        key_sequences = [key for keylist in self.currentKeySequences
+                             for key in keylist]
         if not keys.endswith('>'):
             self.showerror(title, parent=self,
                            message='Missing the final Key')
@@ -267,7 +267,7 @@
             msg = 'The shift modifier by itself may not be used with'\
                   ' this key symbol.'
             self.showerror(title=title, parent=self, message=msg)
-        elif keySequence in self.currentKeySequences:
+        elif keys in key_sequences:
             msg = 'This key combination is already in use.'
             self.showerror(title=title, parent=self, message=msg)
         else: