Cannot add to list of known types, another type with same contract

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I have a data contract that has a data member of ISet<MyType>, this may be
return as two different collections types. I get the below error when
setting both collection types up as known types.

Type 'NHibernate.Collection.Generic.PersistentGenericSet`1[MyType]' cannot
be added to list of known types since another type
'Iesi.Collections.Generic.HashedSet`1[MyType]' with the same data contract
name 'http://schemas.datacontract.org/2004/07/MyNamespace:ArrayOfMyType' is
already present.

The MSDN docs seems to indicate that this is not allowed. See
http://msdn.microsoft.com/en-us/library/aa347850.aspx

"During serialization, only one type can be known in any given scope for a
given data contract, and equivalent collections all have the same data
contracts. This means that, in the previous example, you cannot add both
ArrayList and Array of Object to known types at the same scope. Again, this
is equivalent to known types behavior for non-collection types, but it is
especially important to understand for collections."

However this knowledge base entry
http://support.microsoft.com/kb/967339
seems to call a very similar scenario a bug.

Not sure if my case falls under this bug or not. Any advise or experience
with this issue?
.