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

Complete previous patch.
diff --git a/Lib/idlelib/NEWS.txt b/Lib/idlelib/NEWS.txt
index d57ba7e..8381209 100644
--- a/Lib/idlelib/NEWS.txt
+++ b/Lib/idlelib/NEWS.txt
@@ -3,6 +3,8 @@
 ======================================
 
 
+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.