Hi!
It doesn't conform to specification, but I wouldn't like to change this behavior. Serializing the same point two times is obviously excessive for polygons. Of course, it's not much work to implement this serialization, but it seems to me that in most cases you'll need to discard that additional point after deserialization anyway. And if you are writing your own importer (rather than using some strict existing one) it shouldn't be a problem, should it?
I said that I wouldn't like to change it, but it doesn't mean that I won't. This whole GeoJSON thing is work in progress, so how it will end up working depends on users feedback and my own experience with it. Any notes, suggestions and arguments are welcome!