EntityAttribute metadata filter generates duplicate attributes

Added duplicate detection to the EntityAttributes filter to prevent creation of Attribute. I left values alone for now. This is much harder given the possibility of complex values, even though simple strings may be the common case. I think it's not worth bothering given that it shouldn't hurt much and doesn't make the XML as visibly ugly as the duplicate Attributes do.

Algorithm filter is fixed. The duplicate detection is primitive but for the filter's use cases just comparing by algorithm should be the right answer.

NameIDFormat filter is fixed. The others are much tougher.

Yes, that in fact is how I detected it. I love the new mdquery functionality that lets me see the metadata after receipt and processing by the IdP. And I was testing my entity attributes using it when I discovered this issue.

I'm guessing this as actually noticed as a consequence of querying the IdP for metadata. I hadn't thought about it but that's a good means of verifying the filter behavior, which makes a little more inclined to see if this is fixable without undue effort.