oil in my water feeder bottle in a 1998 renault megane scenic?

Can anyone advise me as to why I have got pure oil in my 1998 Renault Megane scenic water feeder bottle?I had my head gasket done in early 2010,to the find oil again in my feeder bottle Sept/Oct 2010 my garage replaced the head gasket once again and I have again found oil in my water bottle.The garage has had the head checked and was found to be ok.