bug#34987: 27.0.50; Error after toggling duplicate suppression

[Basil L. Contovounesios]

Further to the discussion in bug#34973 and bug#34974, it indeed looks to
me like gnus-suppress-duplicates cannot be enabled only after entering
the summary buffer.  In other words, I see the following:

0. Start Gnus with gnus-suppress-duplicates left at default value of nil
1. Enter summary buffer
2. M-x set-variable RET gnus-suppress-duplicates RET t RET
3. M-u

Debugger entered--Lisp error: (wrong-type-argument hash-table-p nil)
  remhash("<redacted-message-id>" nil)
  gnus-dup-unsuppress-article(36322)
  gnus-mark-article-as-unread(36322 32)
  gnus-summary-mark-article(nil 32 nil)
  gnus-summary-mark-forward(1 32)
  gnus-summary-clear-mark-forward(1)
  funcall-interactively(gnus-summary-clear-mark-forward 1)
  call-interactively(gnus-summary-clear-mark-forward nil nil)
  command-execute(gnus-summary-clear-mark-forward)

Patch to follow.

Thanks,

-- 
Basil

Gnus v5.13
In GNU Emacs 27.0.50 (build 6, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-03-25 built on thunk
Repository revision: 5fdf4fc07df7dd897931efb5dbf5f26dfaff9274
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Debian GNU/Linux buster/sid