[Haskell] duplicate elements in Data.Set

Hello list. Are there situations where a set can contain duplicate elements? I have a newtype and it is an instance of 'Eq' via the 'compare' method, and it is also an instance of 'Ord'. After some Data.Set operations with sets of that type I get a set which contains two elements which compare equal. What am I doing wrong? The function 'Set.valid' returns 'False' when applied to the set. I use the function 'Set.fromList' sometimes. Is it supposed to always yield a valid set? or it may depend on the order or equality? Sorry the question is a little vague. When I try to construct small examples the problem disappear. Thanks, Walter