diff --git a/common.py b/common.py
index f6b19cf..493b14c 100644
--- a/common.py
+++ b/common.py
@@ -18,8 +18,6 @@
 
 """
 Common stuff.
-
-$Id$
 """
 
 from zope import component
@@ -350,23 +348,24 @@ class RelationSet(object):
 
     @Lazy
     def predicate(self):
-        #return self.conceptManager[self.predicateName]
         return self.conceptManager.get(self.predicateName)
 
 
 class ParentRelationSet(RelationSet):
 
     def add(self, related, order=0, relevance=1.0):
-        #if isinstance(related, AdapterBase):
-        #    related = related.context
         related = baseObject(related)
+        existing = list(self.context.getParentRelations([self.predicate], related,
+                        noSecurityCheck=self.noSecurityCheck))
+        if len(existing) == 1:
+            rel = existing[0]
+            if rel.order == order and rel.relevance == relevance:
+                return
         self.context.deassignParent(related, [self.predicate],  # avoid duplicates
                                     noSecurityCheck=self.noSecurityCheck)
         self.context.assignParent(related, self.predicate, order, relevance)
 
     def remove(self, related):
-        #if isinstance(related, AdapterBase):
-        #    related = related.context
         self.context.deassignParent(baseObject(related), [self.predicate])
 
     def __iter__(self):
@@ -396,8 +395,13 @@ class ParentRelationSet(RelationSet):
 class ChildRelationSet(RelationSet):
 
     def add(self, related, order=0, relevance=1.0):
-        if isinstance(related, AdapterBase):
-            related = related.context
+        related = baseObject(related)
+        existing = list(self.context.getChildRelations([self.predicate], related,
+                        noSecurityCheck=self.noSecurityCheck))
+        if len(existing) == 1:
+            rel = existing[0]
+            if rel.order == order and rel.relevance == relevance:
+                return
         self.context.deassignChild(related, [self.predicate])   # avoid duplicates
         self.context.assignChild(related, self.predicate, order, relevance)