bug#15537: 24.3; Man-completion-cache duplicates entries

[Roland Winkler]

start emacs -Q
M-x man
Request alternatingly minibuffer completion for "ld" and "ls".

Expected result: each possible completion appears only once in
Man-completion-cache.

Actual result: Each time I switch from requesting completion for one
string to requesting completion for the other string, this adds once
more all possible completions to Man-completion-cache.  So the cache
keeps growing with duplicate entries.  How is the car of
Man-completion-cache supposedd to help?


In GNU Emacs 24.3.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.4.2)
 of 2013-10-04 on regnitz
Windowing system distributor `The X.Org Foundation', version 11.0.11103000
System Description:     Ubuntu 12.04.3 LTS