Refer StackOverflow question (http://stackoverflow.com/q/21227428/30007) for detailed description. The answer to that question also suggests a workaround in the meanwhile.
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var update = new UpdateBuilder<SomeClass>();
update.Set(x => x.Prop1, value1);
update.Set(x => x.Prop2, value2);
update.Set(x => x.Prop1, anotherValue1); // this throws a Duplicate element name error. It should overwrite the value of Prop1 with anotherValue1.