An item with the same key has already been added.

[B1nke]

Don't know if this should be regarded as a bug, but if the viewmodel contains two properties with the same name, but in different case, when the modelbinder adds propertynames, this error is thrown.
If this will not be supported, maby a little mer explanatory exception should be thrown, so one knows that it's actually the vm that contains an error, and not the markup (as I thought).