"The resource MultipleIdDefinition was not found" on deserialization on Windows Phone 7 - silverlight

Question

I suddenly started getting this from my Windows Phone 7 app. I don't know what changed in the data I was deserialization that would have caused this. Any idea what it means?
System.Runtime.Serialization.SerializationException occurred
Message=The resource MultipleIdDefinition was not found in System.Runtime.Serialization
StackTrace:
at System.Runtime.Serialization.ReferencedObjectCache.Add(String id, Object referencedObject)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.AddNewObjectWithId(String id, Object obj, Boolean isReference)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.AddNewObject(Object obj, Boolean isReference)
at System.Runtime.Serialization.XmlFormatReader.InvokeBeforeDeserialization(ClassDataContract classContract, XmlObjectSerializerReadContext context)
at System.Runtime.Serialization.XmlFormatReader.InitializeCallStack(DataContract clContract, XmlReaderDelegator xmlReaderDelegator, XmlObjectSerializerReadContext xmlObjContext, XmlDictionaryString[] memberNamesColl, XmlDictionaryString[] memberNamespacesColl)
at System.Runtime.Serialization.ClassDataContract.ReadXmlValue(XmlReaderDelegator xmlReader, XmlObjectSerializerReadContext context)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.ReadDataContractValue(DataContract dataContract, XmlReaderDelegator reader)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator reader, String name, String ns, DataContract& dataContract)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator xmlReader, Int32 id, RuntimeTypeHandle declaredTypeHandle, String name, String ns)
at System.Runtime.Serialization.XmlFormatReader.InternalDeserialize(Object value, Type type, String name, String ns, XmlObjectSerializerReadContext context, XmlReaderDelegator xmlReader)
at System.Runtime.Serialization.XmlFormatReader.ReadValue(Type type, String name, String ns, XmlObjectSerializerReadContext context, XmlReaderDelegator xmlReader)
at System.Runtime.Serialization.XmlFormatReader.ReadCollectionItem(CollectionDataContract collectionContract, Type itemType, String itemName, String itemNs, XmlObjectSerializerReadContext context, XmlReaderDelegator xmlReader)
at System.Runtime.Serialization.XmlFormatReader.ReadCollectionItems(DeserializedObject desObj, CollectionDataContract collectionContract, Int32 membersRead, Int32 size)
at System.Runtime.Serialization.XmlFormatReader.ReadCollection(DeserializedObject desObj, CollectionDataContract collectionContract, Int32 membersRead)
at System.Runtime.Serialization.XmlFormatReader.Deserialize(XmlObjectSerializerReadContext context)
at System.Runtime.Serialization.XmlFormatReader.InitializeCallStack(DataContract clContract, XmlReaderDelegator xmlReaderDelegator, XmlObjectSerializerReadContext xmlObjContext, XmlDictionaryString[] memberNamesColl, XmlDictionaryString[] memberNamespacesColl)
at System.Runtime.Serialization.ClassDataContract.ReadXmlValue(XmlReaderDelegator xmlReader, XmlObjectSerializerReadContext context)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.ReadDataContractValue(DataContract dataContract, XmlReaderDelegator reader)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator reader, String name, String ns, DataContract& dataContract)
at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator xmlReader, Type declaredType, DataContract dataContract, String name, String ns)
at System.Runtime.Serialization.DataContractSerializer.InternalReadObject(XmlReaderDelegator xmlReader, Boolean verifyObjectName)
at System.Runtime.Serialization.XmlObjectSerializer.ReadObjectHandleExceptions(XmlReaderDelegator reader, Boolean verifyObjectName)
at System.Runtime.Serialization.XmlObjectSerializer.ReadObject(XmlDictionaryReader reader)
at ...
at Microsoft.Phone.Shell.PhoneApplicationService.FireLaunching()
at Microsoft.Phone.Execution.NativeEmInterop.FireOnLaunching()

Answer 1

If you're not knowlingly deserializing any XML, then I can only guess that either:
the app's isolated storage contains some app-state XML (e.g. from AppSettings)
or that there's a problem with the configuration XML for your app
To try to fix this, you could:
Take a look in the Properties folder of your project - are the XML configuration files there OK? If you open them in the editor, are there any warnings or errors shown?
Uninstall the app from the emulator and/or from your phone (you can even try restarting the emulator or factory resetting the phone)
If not, post more info and we'll try to help more - interesting problem!

Answer 2

If you are working with WCF, this might be caused by added "IsReference=true" of the DataContract at server side.
In actual payload it will be noticeable as something like z:Id=”i7”.
If you have two versions (one working and one not working) I would recommend to debug as deep as possible so you can clearly see stuff you deserialize, and then do text comparison of two versions.