I had the same error today and the problem was caused by the fact that there were two methods with the same name in the XSoapBindingImpl class. I didn't think this would cause a problem as I only defined one method signature in the service interface.
If I viewed the wsdl through a web browser there was no mention of the second method but when I added it as a web reference through .NET it merged all the parameters from both method calls under the same operation using a : to divide them.
When I deleted the method that I didn't want exposed (from the XSoapBindingImpl class) and regenerated everything, this problem no longer existed and I was able to add the web reference successfully.
I can't call the web service but that's a different thread.
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