In Emacs 30.1, completion-preview and yasnippet appear to conflict with
each other. In a c-mode buffer with eglot, yasnippet, and
completion-preview enabled, accepting a completion from
completion-preview with M-i inserts the formal parameters over and over
again.

        // Initial state -- cursor is at the pipe (|);
        int add(int x, int y) { return x + y; }
        int f(void) { |

        // Type "add"; completion-preview activates and proposes a
        // completion (the text `(int x, int y)` is not actual text,
        // but an overlay in a lighter color and underlined.)
        int add(int x, int y) { return x + y; }
        int f(void) { add|(int x, int y)

        // Pressing M-i to accept the completion moves the cursor
        // inside the parentheses, but confusingly creates a copy
        // of the parameters outside the parentheses.
        int add(int x, int y) { return x + y; }
        int f(void) { add(|int x, int y)int x, int y)

        // Pressing M-i again adds more text to the end of the line
        int add(int x, int y) { return x + y; }
        int f(void) { add(|int x, int y)int x, int y), int y)

The following init.el should be sufficient to reproduce the bug.
I used the clangd LSP from the Debian apt repositories.

        (package-initialize)
        (use-package eglot
          :hook (c-mode . eglot-ensure))
        (use-package completion-preview
          :hook (c-mode . completion-preview-mode))
        (use-package yasnippet
          :hook (c-mode . yas-minor-mode))

In GNU Emacs 30.1 (build 3, x86_64-pc-linux-gnu, GTK+ Version 3.24.43,
 cairo version 1.18.2) of 2025-02-26 built on x1
Windowing system distributor 'The X.Org Foundation', version 11.0.12101015
System Description: Debian GNU/Linux trixie/sid

Configured using:
 'configure --prefix=/home/vfoley/.local --with-native-compilation=aot'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG
LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER
PNG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER
WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
  value of $LC_ALL: en_US.utf8
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

Major mode: C/*l

Minor modes in effect:
  eglot-inlay-hints-mode: t
  eglot--managed-mode: t
  flymake-mode: t
  completion-preview-mode: t
  yas-minor-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  minibuffer-regexp-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  abbrev-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message yank-media puny dired
dired-loaddefs rfc822 mml mml-sec epa derived epg rfc6068 epg-config
gnus-util 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 comp-run comp-common rx eglot
external-completion jsonrpc xref flymake thingatpt project compat diff
diff-mode track-changes easy-mmode ert pp ewoc debug backtrace find-func
filenotify warnings compile text-property-search comint ansi-osc
ansi-color ring pcase imenu completion-preview yasnippet cc-mode
cc-fonts cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars
cc-defs cl-extra help-mode use-package-core finder-inf
yasnippet-autoloads package browse-url url url-proxy url-privacy
url-expand url-methods url-history url-cookie generate-lisp-file
url-domsuf url-util mailcap url-handlers url-parse auth-source cl-seq
eieio eieio-core cl-macs icons password-cache json subr-x map byte-opt
gv bytecomp byte-compile url-vars cl-loaddefs cl-lib rmc iso-transl
tooltip cconv eldoc paren electric uniquify ediff-hook vc-hooks
lisp-float-type elisp-mode mwheel term/x-win x-win term/common-win x-dnd
touch-screen 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
nadvice seq simple cl-generic indonesian philippine 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 abbrev obarray oclosure
cl-preloaded button loaddefs theme-loaddefs faces cus-face macroexp
files window text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget keymap hashtable-print-readable backquote
threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting cairo gtk x-toolkit xinput2 x multi-tty move-toolbar
make-network-process native-compile emacs)

Memory information:
((conses 16 254279 18736) (symbols 48 15078 0) (strings 32 66912 2838)
 (string-bytes 1 2272245) (vectors 16 41013)
 (vector-slots 8 418802 16721) (floats 8 88 172) (intervals 56 1405 0)
 (buffers 992 18))