src/Pure/Admin/build_log.scala

       db.transaction {
         db.create_table(table)
 
         val key = Meta_Info.log_name
         val known_files =
-          using(db.select(table, List(key)))(stmt =>
+          using(db.select(table, List(key), distinct = true))(stmt =>
             SQL.iterator(stmt.executeQuery)(rs => db.string(rs, key)).toSet)
 
         val unique_files =
           (Map.empty[String, JFile] /: files.iterator)({ case (m, file) =>
             val name = Log_File.plain_name(file.getName)