diff --git a/briar-core/src/org/briarproject/messaging/SubscriptionUpdateReader.java b/briar-core/src/org/briarproject/messaging/SubscriptionUpdateReader.java
index f9db08a2668ae8f8a851c6f2959f3cab1ce73e00..983be4a049d0fb9e82fc37d2c465c6576d32cff9 100644
--- a/briar-core/src/org/briarproject/messaging/SubscriptionUpdateReader.java
+++ b/briar-core/src/org/briarproject/messaging/SubscriptionUpdateReader.java
@@ -7,10 +7,13 @@ import static org.briarproject.api.messaging.Types.SUBSCRIPTION_UPDATE;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 import org.briarproject.api.FormatException;
 import org.briarproject.api.messaging.Group;
+import org.briarproject.api.messaging.GroupId;
 import org.briarproject.api.messaging.SubscriptionUpdate;
 import org.briarproject.api.serial.Consumer;
 import org.briarproject.api.serial.CountingConsumer;
@@ -31,11 +34,15 @@ class SubscriptionUpdateReader implements StructReader<SubscriptionUpdate> {
 		r.addConsumer(counting);
 		// Read the start of the struct
 		r.readStructStart(SUBSCRIPTION_UPDATE);
-		// Read the subscriptions
+		// Read the subscriptions, rejecting duplicates
 		List<Group> groups = new ArrayList<Group>();
+		Set<GroupId> ids = new HashSet<GroupId>();
 		r.readListStart();
-		for(int i = 0; i < MAX_SUBSCRIPTIONS && !r.hasListEnd(); i++)
-			groups.add(groupReader.readStruct(r));
+		for(int i = 0; i < MAX_SUBSCRIPTIONS && !r.hasListEnd(); i++) {
+			Group g = groupReader.readStruct(r);
+			if(!ids.add(g.getId())) throw new FormatException(); // Duplicate
+			groups.add(g);
+		}
 		r.readListEnd();
 		// Read the version number
 		long version = r.readInteger();