bpo-39600: Adjust code, add idlelib/NEWS item (GH-18449)

Complete previous patch.

Author: terryjreedy

Lib/idlelib/NEWS.txt

@@ -3,6 +3,8 @@ Released on 2020-10-05?
 ======================================
 
 
+bpo-39600: Remove duplicate font names from configuration list.
+
 bpo-38792: Close a shell calltip if a :exc:`KeyboardInterrupt`
 or shell restart occurs.  Patch by Zackery Spytz.
 

Lib/idlelib/configdialog.py

@@ -606,10 +606,8 @@ def load_font_cfg(self):
         font_size = configured_font[1]
         font_bold  = configured_font[2]=='bold'
 
-        # Set editor font selection list and font_name.
-        fonts = tkFont.families(self)
-        # remove duplicated names and sort
-        fonts = sorted(set(fonts))
+        # Set sorted no-duplicate editor font selection list and font_name.
+        fonts = sorted(set(tkFont.families(self)))
         for font in fonts:
             self.fontlist.insert(END, font)
         self.font_name.set(font_name)