"Item has already been added. Key in dictionary:" calling a ws met

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I have two projects (one using .NET 1.1 the other using 2.0). When I call a
particular method in a webservice from the 1.1 project everything works
great. When I call the same method from the 2.0 project (identical code) it
throws an exception with the message:

"{"Item has already been added. Key in dictionary:
'AutoCompleteBackOrders.SopranoWebServices.BaseReq uestDTO' Key being added:
'AutoCompleteBackOrders.SopranoWebServices.BaseReq uestDTO'"}
The stack trace looks like this:
" at System.Xml.Serialization.XmlSerializer.Serialize(X mlWriter xmlWriter,
Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String
id)\r\n at System.Xml.Serialization.XmlSerializer.Serialize(X mlWriter
xmlWriter, Object o, XmlSerializerNamespaces namespaces, String
encodingStyle)\r\n at
System.Web.Services.Protocols.SoapHttpClientProtoc ol.Serialize(SoapClientMessage
message)\r\n at
System.Web.Services.Protocols.SoapHttpClientProtoc ol.Invoke(String
methodName, Object[] parameters)\r\n at
AutoCompleteBackOrders.SopranoWebServices.SopranoW ebServices.psidDetermination(PsidDeterminationRequ estDTO
psidDeterminationRequestDTO) in
C:\\projects\\MOAB_2007_MAR\\BusinessProcessUIs\\T NWCT\\src\\AutoCompleteBackOrder\\AutoCompleteBack order\\Web
References\\SopranoWebServices\\Reference.cs:line 439\r\n at
AutoCompleteBackOrders.DataHandler_Soprano.getPsid Determination(String
busOrgId, String productId, String userName) in
C:\\projects\\MOAB_2007_MAR\\BusinessProcessUIs\\T NWCT\\src\\AutoCompleteBackOrder\\AutoCompleteBack order\\Datahandlers\\DataHandler_Soprano.cs:line 37"
Anybody have any guesses?

--
Kevin Schneider