bug#29323: kill-do-not-save-duplicate, FR

[Drew Adams <drew.adams@oracle.com>]

> >> Currently variable kill-do-not-save-duplicates checks only the (car
> >> kill-ring) as docu explains: Do not add a new string to ‘kill-ring’
> >> if it duplicates the last one. The comparison is done using
> >> ‘equal-including-properties’. AFAIU it would be trivial replace this
> >> check by a call of "member", thus checking > the whole kill-ring.
> >
> > Why do that? Why not just prevent duplicates in the first place, which
> > is what the option currently does? If you for some reason get
> > duplicate entries somehow, in spite of using the option to prevent
> > them, you can always remove them. I don't understand how that would be
> > something that would happen normally. What is the problem that this
> > would try to solve/prevent? ---
> 
> Currently not a check for duplicates is implemented, but for a repeat.
> 
> When having alternating strings to copy, they go into the kill-ring one
> after one. That way it ended up having just two strings in the
> kill-ring, and previous content lost.

Sorry; my bad.  I don't use that variable.  Clearly you
are right.  The option should perhaps have 3 values: one
to do nothing special, one to not push when the car is
the same, and one to not push when the same is on the
right somewhere.

> BTW implementing it would be a way more complicated as thought because
> of text properties.

Not a big deal, I think.  It just uses predicate
`equal-including-properties', which is coded in C.

But now that this has come up... Perhaps the predicate
to test equality should be the value of a variable, to
give users the ability to control the behavior better.

Or barring that (which I'd prefer), perhaps it could
at least let a user choose whether to distinguish
entries if they are the same other than their properties.

> > BTW - it's a pain to remove all of the formatting of your mails to
> > such lists.
> 
> Hmm, don't you see a formatting when sending.
> 
> > Please consider using plain text, or at a minimum not using a colored
> > (i.e. non-white) background.
> 
> Switched on "Readers Default Colors", which should help.

Whatever you're doing now works, for me at least.  Thanks.