diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index efab62fd85..578a0a20e1 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -376,7 +376,8 @@ static void shortlog(const char *name,
 			string_list_append(&subjects,
 					   oid_to_hex(&commit->object.oid));
 		else
-			string_list_append(&subjects, strbuf_detach(&sb, NULL));
+			string_list_append_nodup(&subjects,
+						 strbuf_detach(&sb, NULL));
 	}
 
 	if (opts->credit_people)