diff --git a/lib.php b/lib.php
index e710abf..a928cbd 100644
--- a/lib.php
+++ b/lib.php
@@ -692,18 +692,23 @@ public function restore_user_enrolment(
      * 2. If all duplicates are inactive user records, then use the most recently created one.
      * 3. If there is a mix between active and inactive user records, then use the most recently created active one.
      * @param array $users An array of Ilios user records.
+     * @param progress_trace $trace A logger ("trace") object.
      * @return array The given array of Ilios user records, with duplicates removed.
      */
-    protected function deduplicate_ilios_users(array $users): array {
+    protected function deduplicate_ilios_users(array $users, progress_trace $trace): array {
         // Reverse-sort users by their user ID. In other words, from most-recently created to oldest.
         array_multisort(array_column($users, 'id'), SORT_DESC, SORT_NUMERIC, $users);
         $cache = [];
+        $duplicatekeys = [];
         foreach ($users as $user) {
             $key = $user->campusId;
             // If this is the first time we encounter this user, cache them and move on.
             if (!array_key_exists($key, $cache)) {
                 $cache[$key] = $user;
                 continue;
+            } else {
+                // Track the duplicate campus id.
+                $duplicatekeys[] = $key;
             }
 
             // And now we're dealing with duplicates.
@@ -714,6 +719,15 @@ protected function deduplicate_ilios_users(array $users): array {
             }
         }
 
+        // Log the duplicate campus IDs.
+        if (!empty($duplicatekeys)) {
+            sort($duplicatekeys);
+            $trace->output(
+                'Duplicate Ilios user records found, with the following campus IDs: '
+                . implode(', ', array_unique($duplicatekeys))
+            );
+        }
+
         return array_values($cache);
     }
 }
diff --git a/tests/lib_test.php b/tests/lib_test.php
index 26ec22b..f15ef2d 100644
--- a/tests/lib_test.php
+++ b/tests/lib_test.php
@@ -2142,6 +2142,11 @@ public function test_sync_deduplication_of_ilios_users_by_campus_id(): void {
             "unassigning role: {$user5->id} ==> {$course->id} as {$studentrole->shortname}",
             $output,
         );
+        $this->assertStringContainsString(
+            'Duplicate Ilios user records found, with the following campus IDs: '
+            . 'xx1000001, xx1000002, xx1000003, xx1000004, xx1000005',
+            $output
+        );
 
         // Check user enrollments and role assignments post-sync.
         // Users 1-4 should still be actively enrolled as students.
@@ -2201,6 +2206,11 @@ public function test_sync_deduplication_of_ilios_users_by_campus_id(): void {
         $trace->reset_buffer();
 
         // Check the logging output.
+        $this->assertStringContainsString(
+            'Duplicate Ilios user records found, with the following campus IDs: '
+            . 'xx1000001, xx1000002, xx1000003, xx1000004, xx1000005',
+            $output
+        );
         // There should be nothing in there pertaining to new (un-)enrollments nor role (un-)assignments.
         $this->assertStringNotContainsString('unenrolling:', $output);
         $this->assertStringNotContainsString('unassigning role:', $output);