diff --git a/includes/CommentParser.php b/includes/CommentParser.php
index 59049cc4d..b630c71c8 100644
--- a/includes/CommentParser.php
+++ b/includes/CommentParser.php
@@ -1149,7 +1149,7 @@ class CommentParser {
 
 		if ( isset( $this->threadItemsById[$id] ) ) {
 			// Well, that's tough
-			$threadItem->addWarning( 'Duplicate comment ID' );
+			$threadItem->addWarning( 'Duplicate comment legacy ID' );
 			// Finally, disambiguate by adding sequential numbers, to allow replying to both comments
 			$number = 1;
 			while ( isset( $this->threadItemsById["$id|$number"] ) ) {
diff --git a/tests/phpunit/CommentParserTest.php b/tests/phpunit/CommentParserTest.php
index 2703194f1..79e498d31 100644
--- a/tests/phpunit/CommentParserTest.php
+++ b/tests/phpunit/CommentParserTest.php
@@ -89,6 +89,8 @@ class CommentParserTest extends IntegrationTestCase {
 		$serialized->id = $threadItem->getId();
 
 		$serialized->warnings = $threadItem->getWarnings();
+		// Ignore warnings about legacy IDs (we don't have them in JS)
+		$serialized->warnings = array_values( array_diff( $serialized->warnings, [ 'Duplicate comment legacy ID' ] ) );
 
 		$serialized->replies = [];
 		foreach ( $threadItem->getReplies() as $reply ) {