src/Pure/Thy/sessions.scala

     pos: Position.T = Position.none,
     doc_names: List[String] = Nil,
     global_theories: Map[String, String] = Map.empty,
     loaded_theories: Graph[String, Outer_Syntax] = Graph.string,
     used_theories: List[((Document.Node.Name, Options), List[Document.Node.Name])] = Nil,
-    dump_checkpoint: List[Document.Node.Name] = Nil,
+    dump_checkpoints: Set[Document.Node.Name] = Set.empty,
     known: Known = Known.empty,
     overall_syntax: Outer_Syntax = Outer_Syntax.empty,
     imported_sources: List[(Path, SHA1.Digest)] = Nil,
     sources: List[(Path, SHA1.Digest)] = Nil,
     session_graph_display: Graph_Display.Graph = Graph_Display.empty_graph,

     def get(name: String): Option[Base] = session_bases.get(name)
 
     def imported_sources(name: String): List[SHA1.Digest] =
       session_bases(name).imported_sources.map(_._2)
 
-    def dump_checkpoint: List[Document.Node.Name] =
+    def dump_checkpoints: Set[Document.Node.Name] =
       (for {
         (_, base) <- session_bases.iterator
-        name <- base.dump_checkpoint.iterator
-      } yield name).toList
+        name <- base.dump_checkpoints.iterator
+      } yield name).toSet
 
     def used_theories_condition(default_options: Options, progress: Progress = No_Progress)
       : Graph[Document.Node.Name, Options] =
     {
       val default_skip_proofs = default_options.bool("skip_proofs")

               progress.echo("Session " + info.chapter + "/" + info.name + groups)
             }
 
             val dependencies = resources.session_dependencies(info)
 
-            val dump_checkpoint = resources.dump_checkpoint(info)
+            val dump_checkpoints = resources.dump_checkpoints(info)
 
             val overall_syntax = dependencies.overall_syntax
 
             val theory_files = dependencies.theories.map(_.path)
             val loaded_files =

                 pos = info.pos,
                 doc_names = doc_names,
                 global_theories = global_theories,
                 loaded_theories = dependencies.loaded_theories,
                 used_theories = used_theories,
-                dump_checkpoint = dump_checkpoint,
+                dump_checkpoints = dump_checkpoints,
                 known = known,
                 overall_syntax = overall_syntax,
                 imported_sources = check_sources(imported_files),
                 sources = check_sources(session_files),
                 session_graph_display = session_graph_display,