Except that they are both original sources. Two separate sites can come up with similar articles, especially if they collaborated with one another on it. The fact that both articles link to one another with one being described as easier to digest and one being more thorough implies that this is what happened here. Therefore, neither would be blogspam, just different approaches to the same topic. They both have their merits, one is easier to digest, one is a lot more detailed. If someone is interested in the other, they can easily get to it through the links in the respective articles.

The submitted (Google Plus) link was published May 17th. The more detailed (mathistopheles) link was published May 5th and was only updated very recently to link to the summarized Google Plus post. It seems clear to me that the Google Plus post summarized the mathistopheles post.

I do think that the word blogspam is too strong in this case, but given that the mathistopheles post is the original one by almost several weeks, by convention that should have been submitted as the original source.

If it is of any relevance, I am the original poster and I have no objection to Richard Green's article being the one referenced. He has clearly credited me as his source, and very few people would have seen my piece if he hadn't written his. Also his piece is clearly much more digestible in summarising the points of interest, while mine was more of a puzzle (the solution page is the one being referenced) followed by a loose account of my investigation into the problem.

I'm just pleased that it seems to have captured the imagination in the way it has.