diff --git a/cmscontrib/DumpExporter.py b/cmscontrib/DumpExporter.py
index d6a06790c7..667af71878 100755
--- a/cmscontrib/DumpExporter.py
+++ b/cmscontrib/DumpExporter.py
@@ -183,9 +183,6 @@ def __init__(
                 )
                 self.tasks_ids = [task.id for task in tasks]
         else:
-            # FIXME: this is ATM broken, because if you export a contest, you
-            # then export the users who participated in it and then all of the
-            # contests those users participated in.
             self.contests_ids = contest_ids
             self.users_ids = []
             self.tasks_ids = []
@@ -376,6 +373,10 @@ class of the given object), an item for each column property
             elif isinstance(val, other_cls):
                 data[prp.key] = self.get_id(val)
             elif isinstance(val, list):
+                # Shared users and teams must not pull in other contests.
+                if other_cls is Participation:
+                    val = [i for i in val
+                           if i.contest_id in self.contests_ids]
                 data[prp.key] = list(self.get_id(i) for i in val)
             elif isinstance(val, dict):
                 data[prp.key] = \
diff --git a/cmscontrib/DumpImporter.py b/cmscontrib/DumpImporter.py
index 1096ef56b1..8566dd63eb 100755
--- a/cmscontrib/DumpImporter.py
+++ b/cmscontrib/DumpImporter.py
@@ -64,6 +64,8 @@
     FilenameSchemaArray,
     Digest,
     SessionGen,
+    Admin,
+    Team,
     Contest,
     Submission,
     SubmissionResult,
@@ -262,9 +264,49 @@ def do_import(self):
 
                 assert self.datas["_version"] == model_version
 
+                # Only root contests were selected for export; shared users
+                # and teams can also reference unrelated contests in old dumps.
+                contest_keys = {key for key in self.datas["_objects"]
+                                if self.datas[key]["_class"] == "Contest"}
+                import_keys = set()
+                queue = list(self.datas["_objects"])
+                while queue:
+                    key = queue.pop()
+                    if key in import_keys:
+                        continue
+                    data = self.datas[key]
+                    if ((data["_class"] == "Contest" and key not in contest_keys)
+                            or (data.get("contest") is not None
+                                and data["contest"] not in contest_keys)):
+                        continue
+                    import_keys.add(key)
+                    cls = getattr(class_hook, data["_class"])
+                    for prp in cls._rel_props:
+                        references = data.get(prp.key)
+                        if isinstance(references, str):
+                            queue.append(references)
+                        elif isinstance(references, list):
+                            queue.extend(references)
+                        elif isinstance(references, dict):
+                            queue.extend(references.values())
+
                 self.objs = dict()
+                reused_keys = set()
+                shared_classes = {"Admin": (Admin, "username"),
+                                  "User": (User, "username"),
+                                  "Team": (Team, "code")}
                 for id_, data in self.datas.items():
-                    if not id_.startswith("_"):
+                    if id_ in import_keys:
+                        if data["_class"] in shared_classes:
+                            cls, field = shared_classes[data["_class"]]
+                            existing = session.query(cls).filter(
+                                getattr(cls, field) == data[field]).one_or_none()
+                            if existing is not None:
+                                self.objs[id_] = existing
+                                reused_keys.add(id_)
+                                logger.info("Reusing existing %s %s.",
+                                            data["_class"], data[field])
+                                continue
                         self.objs[id_] = self.import_object(data)
 
                 for k, v in list(self.objs.items()):
@@ -289,7 +331,8 @@ def do_import(self):
                         del self.objs[k]
 
                 for id_, data in self.datas.items():
-                    if not id_.startswith("_") and id_ in self.objs:
+                    # Preserve existing memberships, credentials and permissions.
+                    if id_ in self.objs and id_ not in reused_keys:
                         self.add_relationships(data, self.objs[id_])
 
                 contest_id = list()