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.

Thanks,

David.

In GNU Emacs 28.0.90 (build 3, i686-pc-linux-gnu, GTK+ Version 3.18.9,
cairo version 1.14.6)
 of 2021-12-22 built on newfont
Repository revision: ce7ec828f15446c1218614d083da489c76e9b37d
Repository branch: cl-del-again
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description: Slackware 14.2

Configured using:
 'configure
 PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/pkgconfig:/usr/share/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
LCMS2 LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND
THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: en_US.ISO8859-1
  locale-coding-system: iso-latin-1-unix

Major mode: ELisp/l

Minor modes in effect:
  bug-reference-prog-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t

Load-path shadows:
/home/dfussner/.emacs.d/elpa/transient-20210426.2141/transient hides
/home/dfussner/src/emacs/emacs/lisp/transient

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg rfc6068 epg-config gnus-util rmail
rmail-loaddefs text-property-search mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils time-date help-fns
radix-tree help-mode misearch multi-isearch vc-git diff-mode easy-mmode
vc-dispatcher bug-reference doom-opera-theme doom-themes
doom-themes-base edmacro kmacro server preview-latex auto-loads tex-site
finder-inf info package browse-url url url-proxy url-privacy url-expand
url-methods url-history url-cookie url-domsuf url-util mailcap
url-handlers url-parse auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json subr-x map url-vars seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib iso-transl tooltip eldoc
paren electric uniquify ediff-hook vc-hooks lisp-float-type elisp-mode
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite emoji-zwj charscript charprop case-table
epa-hook jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice
button loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 8 275188 34213)
 (symbols 24 15726 0)
 (strings 16 79597 3815)
 (string-bytes 1 2052213)
 (vectors 8 24263)
 (vector-slots 4 365962 27260)
 (floats 8 208 168)
 (intervals 28 819 0)
 (buffers 564 14)
 (heap 1024 25437 912))