Issue #18830: inspect.getclasstree() no more produces duplicated entries even

when input list contains duplicates.

diff --cc Lib/inspect.py
Simple merge

diff --cc Lib/test/test_inspect.py
Simple merge

diff --cc Misc/NEWS
index 5d6489f8871f0d43a7c66c9226918152eaa1374a,66fc600f294ca3de4c83b4b1965ee5219ffff41f..8b6aa564f7c74eead42e3d2083f6d29b098fa047
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -54,9 -66,9 +54,12 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #18830: inspect.getclasstree() no more produces duplicated entries even
+   when input list contains duplicates.
+ 
 +- Issue #18878: sunau.open now supports the context manager protocol.  Based on
 +  patches by Claudiu Popa and R. David Murray.
 +
  - Issue #18909: Fix _tkinter.tkapp.interpaddr() on Windows 64-bit, don't cast
    64-bit pointer to long (32 bits).