* lisp/gnus/gnus-dup.el: Use lexical-binding

(gnus-dup-list-dirty): Add docstring.
(gnus-dup-open): Allocate gnus-dup-hashtb more conservatively now
that it is no longer an obarray.
(gnus-dup-enter-articles): Fix off-by-one error.
(gnus-dup-suppress-articles): DRY.

For discussion, see thread starting at:
https://lists.gnu.org/archive/html/emacs-devel/2019-03/msg00974.html

lisp/gnus/gnus-dup.el

-;;; gnus-dup.el --- suppression of duplicate articles in Gnus
+;;; gnus-dup.el --- suppression of duplicate articles in Gnus  -*- lexical-binding: t -*-
 
 ;; Copyright (C) 1996-2019 Free Software Foundation, Inc.
 
@@ -57,10 +57,12 @@ seen in the same session."
 
 (defvar gnus-dup-list nil
   "List of seen message IDs, as strings.")
+
 (defvar gnus-dup-hashtb nil
   "Hash table of seen message IDs, for fast lookup.")
 
-(defvar gnus-dup-list-dirty nil)
+(defvar gnus-dup-list-dirty nil
+  "Non-nil if `gnus-dup-list' needs to be saved.")
 
 ;;;
 ;;; Starting and stopping
@@ -80,7 +82,7 @@ seen in the same session."
   (if gnus-save-duplicate-list
       (gnus-dup-read)
     (setq gnus-dup-list nil))
-  (setq gnus-dup-hashtb (gnus-make-hashtable gnus-duplicate-list-length))
+  (setq gnus-dup-hashtb (gnus-make-hashtable))
   ;; Enter all Message-IDs into the hash table.
   (dolist (g gnus-dup-list)
     (puthash g t gnus-dup-hashtb)))
@@ -121,11 +123,13 @@ seen in the same session."
 		 (not (gethash msgid gnus-dup-hashtb)))
 	(push msgid gnus-dup-list)
 	(puthash msgid t gnus-dup-hashtb))))
-  ;; Chop off excess Message-IDs from the list.
-  (let ((end (nthcdr gnus-duplicate-list-length gnus-dup-list)))
+  ;; Remove excess Message-IDs from the list and hash table.
+  (let* ((dups (cons nil gnus-dup-list))
+         (end  (nthcdr gnus-duplicate-list-length dups)))
     (when end
       (mapc (lambda (id) (remhash id gnus-dup-hashtb)) (cdr end))
-      (setcdr end nil))))
+      (setcdr end nil))
+    (setq gnus-dup-list (cdr dups))))
 
 (defun gnus-dup-suppress-articles ()
   "Mark duplicate articles as read."
@@ -137,10 +141,9 @@ seen in the same session."
 	number)
     (dolist (header gnus-newsgroup-headers)
       (when (and (gethash (mail-header-id header) gnus-dup-hashtb)
-		 (gnus-summary-article-unread-p (mail-header-number header)))
-	(setq gnus-newsgroup-unreads
-	      (delq (setq number (mail-header-number header))
-		    gnus-newsgroup-unreads))
+                 (setq number (mail-header-number header))
+                 (gnus-summary-article-unread-p number))
+        (setq gnus-newsgroup-unreads (delq number gnus-newsgroup-unreads))
 	(if (not auto)
 	    (push (cons number gnus-duplicate-mark) gnus-newsgroup-reads)
 	  (push number gnus-newsgroup-expirable)