diff --git a/services/univention-s4-connector/modules/univention/s4connector/s4/__init__.py b/services/univention-s4-connector/modules/univention/s4connector/s4/__init__.py
index 7b2bf8f426..e8c57ffa08 100644
--- a/services/univention-s4-connector/modules/univention/s4connector/s4/__init__.py
+++ b/services/univention-s4-connector/modules/univention/s4connector/s4/__init__.py
@@ -1516,14 +1516,15 @@ class s4(univention.s4connector.ucs):
 			self.__group_cache_ucs_append_member(groupDN, object_ucs['dn'])
 
 	def __group_cache_ucs_append_member(self, group, member):
-		group_lower = group.lower()
 		member_lower = member.lower()
-		ud.debug(ud.LDAP, ud.INFO, "__group_cache_ucs_append_member: Append user %s to UCS group member cache of %s" % (member_lower, group_lower))
-		members = self.group_members_cache_ucs.get(group_lower)
-		if not members:
-			self.group_members_cache_ucs[group_lower] = [member_lower]
-		else:
-			members.append(member_lower)
+		group_lower = group.lower()
+		group_members_lower = self.group_members_cache_ucs.get(group_lower, [])
+		if member_lower not in group_members_lower:
+			ud.debug(ud.LDAP, ud.INFO, "__group_cache_ucs_append_member: Append user %s to UCS group member cache of %s" % (member_lower, group_lower))
+			if group_members_lower:
+				group_members_lower.append(member_lower)
+			else:
+				self.group_members_cache_ucs[group_lower] = [member_lower]
 
 	def group_members_sync_from_ucs(self, key, object):  # object mit s4-dn
 		"""
@@ -1728,12 +1729,17 @@ class s4(univention.s4connector.ucs):
 					if ldap_object_ucs:
 						self.one_group_member_sync_to_ucs(ucs_group_object, object)
 
-				if not self.group_members_cache_con.get(groupDN.lower()):
-					self.group_members_cache_con[groupDN.lower()] = []
 				dn = object['attributes'].get('distinguishedName', [None])[0]
 				if dn:
-					ud.debug(ud.LDAP, ud.INFO, "object_memberships_sync_to_ucs: Append user %s to S4 group member cache of %s" % (dn.lower(), groupDN.lower()))
-					self.group_members_cache_con[groupDN.lower()].append(dn.lower())
+					dn_lower = dn.lower()
+					groupDN_lower = groupDN.lower()
+					group_members_lower = self.group_members_cache_con.get(groupDN_lower, [])
+					if dn_lower not in group_members_lower:
+						ud.debug(ud.LDAP, ud.INFO, "object_memberships_sync_to_ucs: Append user %s to S4 group member cache of %s" % (dn_lower, groupDN_lower))
+						if group_members_lower:
+							group_members_lower.append(dn_lower)
+						else:
+							self.group_members_cache_con[groupDN_lower] = [dn_lower]
 				else:
 					ud.debug(ud.LDAP, ud.INFO, "object_memberships_sync_to_ucs: Failed to append user %s to S4 group member cache of %s" % (object['dn'].lower(), groupDN.lower()))
 
@@ -1790,20 +1796,26 @@ class s4(univention.s4connector.ucs):
 				ud.debug(ud.LDAP, ud.INFO, "one_group_member_sync_from_ucs: User is already member of the group: %s modlist: %s" % (s4_group_object['dn'], ml))
 
 		# The user has been removed from the cache. He must be added in any case
-		ud.debug(ud.LDAP, ud.INFO, "one_group_member_sync_from_ucs: Append user %s to S4 group member cache of %s" % (object['dn'].lower(), s4_group_object['dn'].lower()))
-		if not self.group_members_cache_con.get(s4_group_object['dn'].lower()):
-			self.group_members_cache_con[s4_group_object['dn'].lower()] = []
-		self.group_members_cache_con[s4_group_object['dn'].lower()].append(object['dn'].lower())
+		s4_group_object_dn_lower = s4_group_object['dn'].lower()
+		group_members_lower = self.group_members_cache_con.get(s4_group_object_dn_lower, [])
+		object_dn_lower = object['dn'].lower()
+		if object_dn_lower not in group_members_lower:
+			ud.debug(ud.LDAP, ud.INFO, "one_group_member_sync_from_ucs: Append user %s to S4 group member cache of %s" % (object_dn_lower, s4_group_object_dn_lower))
+			if group_members_lower:
+				group_members_lower.append(object_dn_lower)
+			else:
+				self.group_members_cache_con[s4_group_object_dn_lower] = [object_dn_lower]
 
 	def __group_cache_con_append_member(self, group, member):
-		group_lower = group.lower()
 		member_lower = member.lower()
-		ud.debug(ud.LDAP, ud.INFO, "__group_cache_con_append_member: Append user %s to S4 group member cache of %s" % (member_lower, group_lower))
-		members = self.group_members_cache_con.get(group_lower)
-		if not members:
-			self.group_members_cache_con[group_lower] = [member_lower]
-		else:
-			members.append(member_lower)
+		group_lower = group.lower()
+		group_members_lower = self.group_members_cache_con.get(group_lower, [])
+		if member_lower not in group_members_lower:
+			ud.debug(ud.LDAP, ud.INFO, "__group_cache_con_append_member: Append user %s to S4 group member cache of %s" % (member_lower, group_lower))
+			if group_members_lower:
+				group_members_lower.append(member_lower)
+			else:
+				self.group_members_cache_con[group_lower] = [member_lower]
 
 	def group_members_sync_to_ucs(self, key, object):
 		"""