diff --git a/cms/service/ProxyService.py b/cms/service/ProxyService.py
index 3feed45a43..ab37a1e060 100644
--- a/cms/service/ProxyService.py
+++ b/cms/service/ProxyService.py
@@ -477,6 +477,14 @@ def submission_scored(self, submission_id):
                              "unexistent submission id %s.", submission_id)
                 raise KeyError("Submission not found.")
 
+            # ScoringService sent us a submission of another contest, they
+            # do not know about our contest_id in multicontest setup.
+            if submission.task.contest_id != self.contest_id:
+                logger.debug("Ignoring submission %s of contest %d "
+                             "(this ProxyService considers contest %d only)",
+                             submission.id, submission.task.contest_id, self.contest_id)
+                return
+
             if submission.participation.hidden:
                 logger.info("[submission_scored] Score for submission %d "
                             "not sent because the participation is hidden.",
@@ -489,11 +497,6 @@ def submission_scored(self, submission_id):
                             submission_id)
                 return
 
-            # ScoringService sent us a submission of another contest, they
-            # do not know about our contest_id in multicontest setup.
-            if submission.task.contest_id != self.contest_id:
-                return
-
             # Update RWS.
             for operation in self.operations_for_score(submission):
                 self.enqueue(operation)
@@ -517,6 +520,14 @@ def submission_tokened(self, submission_id):
                              "unexistent submission id %s.", submission_id)
                 raise KeyError("Submission not found.")
 
+            # ScoringService sent us a submission of another contest, they
+            # do not know about our contest_id in multicontest setup.
+            if submission.task.contest_id != self.contest_id:
+                logger.debug("Ignoring submission %s of contest %d "
+                             "(this ProxyService considers contest %d only)",
+                             submission.id, submission.task.contest_id, self.contest_id)
+                return
+
             if submission.participation.hidden:
                 logger.info("[submission_tokened] Token for submission %d "
                             "not sent because participation is hidden.",
@@ -529,11 +540,6 @@ def submission_tokened(self, submission_id):
                             submission_id)
                 return
 
-            # ScoringService sent us a submission of another contest, they
-            # do not know about our contest_id in multicontest setup.
-            if submission.task.contest_id != self.contest_id:
-                return
-
             # Update RWS.
             for operation in self.operations_for_token(submission):
                 self.enqueue(operation)
@@ -556,6 +562,14 @@ def dataset_updated(self, task_id):
             task = Task.get_from_id(task_id, session)
             dataset = task.active_dataset
 
+            # This ProxyService may focus on a different contest, and it should
+            # ignore this update.
+            if task.contest_id != self.contest_id:
+                logger.debug("Ignoring dataset change for task %d of contest %d "
+                             "(this ProxyService considers contest %d only)",
+                             task_id, task.contest.id, self.contest_id)
+                return
+
             logger.info("Dataset update for task %d (dataset now is %d).",
                         task.id, dataset.id)