diff --git a/lisp/net/tramp-cache.el b/lisp/net/tramp-cache.el
index 4ecc804bf20..eedb5a3e9ba 100644
--- a/lisp/net/tramp-cache.el
+++ b/lisp/net/tramp-cache.el
@@ -647,7 +647,7 @@ your laptop to different networks frequently."
   "Return a list of (user host) tuples allowed to access for METHOD.
 This function is added always in `tramp-get-completion-function'
 for all methods.  Resulting data are derived from connection history."
-  (delete-dups
+  (seq-uniq
    (tramp-compat-seq-keep
     (lambda (key)
       (let ((tramp-verbose 0))
diff --git a/lisp/net/tramp-fuse.el b/lisp/net/tramp-fuse.el
index 72b1934faca..07f4ef74581 100644
--- a/lisp/net/tramp-fuse.el
+++ b/lisp/net/tramp-fuse.el
@@ -59,11 +59,10 @@
   (let ((result
 	 (tramp-skeleton-directory-files directory full match nosort count
 	   ;; Some storage systems do not return "." and "..".
-	   (delete-dups
-	    (append
-	     '("." "..")
-	     (tramp-fuse-remove-hidden-files
-	      (directory-files (tramp-fuse-local-file-name directory))))))))
+	   (seq-union
+	    '("." "..")
+	    (tramp-fuse-remove-hidden-files
+	     (directory-files (tramp-fuse-local-file-name directory)))))))
     (if full
 	;; Massage the result.
 	(let ((local (rx
diff --git a/lisp/net/tramp-gvfs.el b/lisp/net/tramp-gvfs.el
index 7f3ac945bb6..c283c119b87 100644
--- a/lisp/net/tramp-gvfs.el
+++ b/lisp/net/tramp-gvfs.el
@@ -2556,7 +2556,7 @@ This uses \"avahi-browse\" in case D-Bus is not enabled in Avahi."
 	   (split-string
 	    (shell-command-to-string (format "avahi-browse -trkp %s" service))
 	    (rx (+ (any "\r\n"))) 'omit (rx bol "+;" (* nonl) eol)))))
-    (delete-dups
+    (seq-uniq
      (tramp-compat-seq-keep
       (lambda (x)
 	(ignore-errors
diff --git a/lisp/net/tramp-sh.el b/lisp/net/tramp-sh.el
index 0a454fed69b..8922adb7586 100644
--- a/lisp/net/tramp-sh.el
+++ b/lisp/net/tramp-sh.el
@@ -2770,7 +2770,7 @@ The method used must be an out-of-band method."
 			 #'file-name-directory (list localname))))
       (unless (or full-directory-p (member "-d" switches))
         (setq switches (append switches '("-d"))))
-      (setq switches (delete-dups switches)
+      (setq switches (seq-uniq switches)
 	    switches (mapconcat #'tramp-shell-quote-argument switches " "))
       (when wildcard
 	(setq switches (concat switches " " wildcard)))
diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index a27e8b8fbe8..e8f791c567b 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -2729,7 +2729,7 @@ This avoids problems during autoload, when `load-path' contains
 remote file names."
   ;; We expect all other Tramp files in the same directory as tramp.el.
   (let* ((dir (expand-file-name (file-name-directory (locate-library "tramp"))))
-	 (files (delete-dups
+	 (files (seq-uniq
 		 (mapcar
 		  #'file-name-sans-extension
 		  (directory-files
@@ -2803,12 +2803,11 @@ whether HANDLER is to be called.  Add operations defined in
   ;; Mark `operations' the handler is responsible for.
   (put #'tramp-file-name-handler
        'operations
-       (delete-dups
-        (append
-         (get 'tramp-file-name-handler 'operations)
-         (mapcar
-          #'car
-          (symbol-value (intern (concat (symbol-name handler) "-alist"))))))))
+       (seq-union
+        (get 'tramp-file-name-handler 'operations)
+        (mapcar
+         #'car
+         (symbol-value (intern (concat (symbol-name handler) "-alist")))))))
 
 (defun tramp-exists-file-name-handler (operation &rest args)
   "Check, whether OPERATION runs a file name handler."
@@ -2979,7 +2978,7 @@ not in completion mode."
 BODY is the backend specific code."
   (declare (indent 2) (debug t))
   `(ignore-error file-missing
-     (delete-dups (delq nil (delete ""
+     (seq-uniq (delq nil (delete ""
        (let* ((case-fold-search read-file-name-completion-ignore-case)
 	      (result (progn ,@body)))
 	 ;; Some storage systems do not return "." and "..".
@@ -3372,7 +3371,7 @@ as for \"~/.authinfo.gpg\"."
 This function is added always in `tramp-get-completion-function'
 for all methods.  Resulting data are derived from default settings."
   (and tramp-completion-use-auth-sources
-       (delete-dups
+       (seq-uniq
 	(tramp-compat-seq-keep
 	 (lambda (x) `(,(plist-get x :user) ,(plist-get x :host)))
 	 (auth-source-search
@@ -3401,7 +3400,7 @@ User is always nil."
       (with-temp-buffer
 	(insert-file-contents-literally filename)
 	(goto-char (point-min))
-	(delete-dups (delq nil
+	(seq-uniq (delq nil
          (cl-loop while (not (eobp)) collect (funcall function))))))))
 
 (defun tramp-parse-rhosts (filename)
@@ -3538,7 +3537,7 @@ Host is always \"localhost\"."
 (defun tramp-parse-netrc (filename)
   "Return a list of (user host) tuples allowed to access.
 User may be nil."
-  (delete-dups
+  (seq-uniq
    (tramp-compat-seq-keep
     (lambda (item)
       (and (assoc "machine" item)
@@ -5347,8 +5346,8 @@ should be set connection-local.")
 	 (or (not (stringp buffer)) (not (tramp-tramp-file-p buffer)))
 	 (or (not (stringp stderr)) (not (tramp-tramp-file-p stderr))))))
 
-;; This function is used by tramp-*-handle-make-process and
-;; tramp-sh-handle-process-file to filter local environment variables
+;; This function is used by `tramp-*-handle-make-process' and
+;; `tramp-sh-handle-process-file' to filter local environment variables
 ;; that should not be propagated remotely.  Users can override this
 ;; function if necessary, for example, to ensure that a specific
 ;; environment variable is applied to remote processes, whether it
diff --git a/test/lisp/net/tramp-archive-tests.el b/test/lisp/net/tramp-archive-tests.el
index 0ff0f3ddd4d..33dc0b9d4af 100644
--- a/test/lisp/net/tramp-archive-tests.el
+++ b/test/lisp/net/tramp-archive-tests.el
@@ -559,7 +559,7 @@ This checks also `file-name-as-directory', `file-name-directory',
 
   (let (;; We test for the summary line.  Keyword "total" could be localized.
 	(process-environment
-	 (append '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
+	 (seq-union '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
     (unwind-protect
 	(progn
 	  (with-temp-buffer
diff --git a/test/lisp/net/tramp-tests.el b/test/lisp/net/tramp-tests.el
index 7b90bb4d9be..e011b99be57 100644
--- a/test/lisp/net/tramp-tests.el
+++ b/test/lisp/net/tramp-tests.el
@@ -3515,7 +3515,7 @@ This tests also `file-directory-p' and `file-accessible-directory-p'."
 	   (tmp-name2 (expand-file-name "foo" tmp-name1))
 	   ;; We test for the summary line.  Keyword "total" could be localized.
 	   (process-environment
-	    (append '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
+	    (seq-union '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
       (unwind-protect
 	  (progn
 	    (make-directory tmp-name1)
@@ -5656,8 +5656,7 @@ If UNSTABLE is non-nil, the test is tagged as `:unstable'."
 (ert-deftest tramp-test30-make-process ()
   "Check `make-process'."
   :tags (append '(:expensive-test :tramp-asynchronous-processes)
-		(and (getenv "EMACS_EMBA_CI")
-                     '(:unstable)))
+		(and (getenv "EMACS_EMBA_CI") '(:unstable)))
   (skip-unless (tramp--test-enabled))
   (skip-unless (tramp--test-supports-processes-p))
 
@@ -7887,8 +7886,8 @@ This requires restrictions of file name syntax."
        "🌈🍒👋")
 
       (when (and (tramp--test-expensive-test-p) (not (tramp--test-windows-nt-p)))
-	(delete-dups
-	 (mapcar
+	(seq-uniq
+	 (tramp-compat-seq-keep
 	  ;; Use all available language specific snippets.
 	  (lambda (x)
 	    (and