diff --git a/Sources/PersonalMessage/Received.php b/Sources/PersonalMessage/Received.php
index 07b2e8eecf..995aff7de6 100644
--- a/Sources/PersonalMessage/Received.php
+++ b/Sources/PersonalMessage/Received.php
@@ -236,16 +236,21 @@ public function save(): void
 			$this->in_inbox = true;
 		}
 
+		// A row here is one member's copy of one PM, and (id_pm, id_member) is
+		// the primary key. Naming only the PM makes this statement try to hand
+		// every recipient's copy to the same member, so anything that saves a
+		// PM with more than one recipient - marking it read, labelling it,
+		// deleting it - is a duplicate key error rather than a save.
 		Db::$db->query(
 			'UPDATE {db_prefix}pm_recipients
 			SET
-				id_member = {int:member},
 				bcc = {int:bcc},
 				is_read = {int:is_read},
 				is_new = {int:is_new},
 				deleted = {int:deleted},
 				in_inbox = {int:in_inbox}
-			WHERE id_pm = {int:id}',
+			WHERE id_pm = {int:id}
+				AND id_member = {int:member}',
 			[
 				'id' => (int) $this->id,
 				'member' => (int) $this->member,
@@ -259,12 +264,22 @@ public function save(): void
 
 		$labels = array_diff($this->labels, [-1]);
 
+		// Labels belong to a member too, so only clear the ones this member
+		// owns. Every other recipient has their own labels on the same PM, and
+		// with the update above fixed this line is finally reached for a PM
+		// that has some.
 		Db::$db->query(
 			'DELETE FROM {db_prefix}pm_labeled_messages
-			WHERE id_pm = {int:current_pm}' . (empty($labels) ? '' : '
+			WHERE id_pm = {int:current_pm}
+				AND id_label IN (
+					SELECT id_label
+					FROM {db_prefix}pm_labels
+					WHERE id_member = {int:member}
+				)' . (empty($labels) ? '' : '
 				AND id_label NOT IN ({array_int:labels})'),
 			[
 				'current_pm' => $this->id,
+				'member' => (int) $this->member,
 				'labels' => $labels,
 			],
 		);