bug#27745: Warn: you have already yanked the same thing 2 (3, 4...) times in a row

[積丹尼 Dan Jacobson]

Unless the user pays attention to the L number in the modeline, and the
shrinking scrollbar, there is no other clues that the user's C-y isn't
stuck or slow, and he is actually probably mistakenly yanking twice...
repeating paragraphs, or even doubling the size of a file.

$ seq 99 > a
$ emacs a
 C-x h [mark-whole-buffer]
 C-w [kill-region]
 C-y [yank]
 C-y [yank]
 C-y [yank]
 C-y [yank]
 C-y [yank]
 C-y [yank]
 C-y [yank]

So maybe print:
  Warning: you have already yanked the same thing 2 (3,4...) times in a row.
if he yanks the exact same thing more than once.

(This problem is similar to
http://debbugs.gnu.org/25433
but that is harder to reproduce, so I'll close it.)