bug#52734: 28.0.90; [PATCH] xref-backend-definitions in etags.el duplicates list

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 22 Dec 2021 15:01:54 +0000
> From:  David Fussner via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> When `etags-xref-prefer-current-file' is t and only the current file
> contains (multiple) definitions of the same function, the list returns
> each tag twice.  To reproduce:
> 
> cd src/
> ../lib-src/etags term.c
> ./emacs -Q term.c
> C-s tty_free_frame_resources RTN
> M-. RTN
> 
> src/term.c
> 3786: tty_free_frame_resources
> 3798: tty_free_frame_resources
> 
> (setq etags-xref-prefer-current-file t)
> M-.
> 
> src/term.c
> 3786: tty_free_frame_resources
> 3798: tty_free_frame_resources
> 3786: tty_free_frame_resources
> 3798: tty_free_frame_resources
> 
> The variable is new in 28.1, so I thought the bug might want fixing on
> this branch.  In my testing, cl-delete appears not to delete ITEMs from
> SEQ if such deletion would leave SEQ empty.  I can't find this behavior
> documented anywhere, but I may be looking in the wrong places.
> 
> Patch attached, which just tests for equality of the two lists of
> definitions, and only returns one of them in that case.

Dmitry, any comments?  The patch LGTM, FWIW.