don't memoize objects that are their own copies (closes #12422)

Patch mostly by Alex Gaynor.
diff --git a/Misc/NEWS b/Misc/NEWS
index cbbc975..bcd597a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -200,6 +200,9 @@
 Library
 -------
 
+- Issue #12422: In the copy module, don't store objects that are their own copy
+  in the memo dict.
+
 - Issue #12303: Add sigwaitinfo() and sigtimedwait() to the signal module.
 
 - Issue #12404: Remove C89 incompatible code from mmap module. Patch by Akira