diff --git a/tests/tp-transaction-tests.el b/tests/tp-transaction-tests.el
index b746820..6174d4b 100644
--- a/tests/tp-transaction-tests.el
+++ b/tests/tp-transaction-tests.el
@@ -99,6 +99,30 @@
    :final-accept #'ignore
    :diagnostics nil))
 
+(ert-deftest tp-transaction-test-unique-identities-use-equal-semantics ()
+  "Distinct identity values stay distinct; equal values remain duplicates."
+  (dolist (items (list nil '(nil) '(1 1.0) '(nil mount-a "mount-a")
+                      (number-sequence 1 2000)))
+    (let ((before (copy-tree items)))
+      (should (tp--proper-unique-list-p items))
+      (should (equal items before))))
+  (dolist (items (list '(nil nil) '(mount-a mount-a)
+                      (list (copy-sequence "mount") (copy-sequence "mount"))
+                      (list (list 'owner 1) (list 'owner 1))
+                      (list (vector 'owner 1) (vector 'owner 1))))
+    (should-not (tp--proper-unique-list-p items))
+    (should-error (tp-transaction-test--entry :mount-ids items)
+                  :type 'tp-publication-binding-error)))
+
+(ert-deftest tp-transaction-test-unique-identities-reject-improper-lists ()
+  "Malformed identity sequences cannot enter publication authority bindings."
+  (let ((cycle (list 'mount-a 'mount-b)))
+    (setcdr (last cycle) cycle)
+    (dolist (items (list 'mount-a [mount-a] '(mount-a . mount-b) cycle))
+      (should-not (tp--proper-unique-list-p items))
+      (should-error (tp-transaction-test--entry :mount-ids items)
+                    :type 'tp-publication-binding-error))))
+
 (defun tp-transaction-test--slot-writes (target values)
   "Return fixed slot writes assigning VALUES into TARGET from index zero."
   (vconcat
diff --git a/tp-transaction.el b/tp-transaction.el
index 7395988..58ec0bb 100644
--- a/tp-transaction.el
+++ b/tp-transaction.el
@@ -83,11 +83,12 @@
 (defun tp--proper-unique-list-p (items)
   "Return non-nil when ITEMS is a proper list with no equal duplicates."
   (and (proper-list-p items)
-       (let (seen (unique t))
+       (let ((seen (make-hash-table :test #'equal))
+             (unique t))
          (dolist (item items unique)
-           (if (member item seen)
+           (if (gethash item seen)
                (setq unique nil)
-             (push item seen))))))
+             (puthash item t seen))))))
 
 (cl-defstruct (tp-publication-target-entry
                (:constructor tp--make-publication-target-entry)