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You probably want to talk to the SEO Moz Support Team directly rather than the Q&A forums - sadly no power here!

I would say that you just need to add rel="canonical" to the on each page - this will tell Google to affectivley ignore all other variations of that URL.

You could also considering using Web Master Tools to advise the search engines to refrain from indexing all of the URLs with those specific parameters - just be careful though as giving the wrong parameter instruction could result in pages you DO want to be indexed being ignored or dropped out of the SERPs.

In-house and freelance in spare time

Sounds like you have been 100% validated in your decision to change SEO firm! Just imagine the tactics they were using when you were paying them

I would echo the comments above - stay vigilant for spammy link warning within Google Webmaster Tools and add any poor quality links that you find to a disavow file.

Its staggering that your old agency would rather put time, money & effort into harming an old client rather than finding a new one

when this happens try appending /logout to the end of the moz URL -(i.e. http://moz.com/logout) this should help you to reload the main page again and then you can login as usual

This would definitely be duplicate content.

Most blogs or websites that welcome guest blog posts stipulate that the content is unique for this very reason.

It has no unique value if it is across numerous websites, it is essentially the same as posting your article on an article distribution website (albeit to a lesser degree).

My tip is to have the article on 1 site only.

Thanks Alan - I was thinking it would be a better solution to propose that a unique article is written by us for them to use on their website (with deep links pointing back at my website).

What do you think?

Why don't you consider looking outside of Mexico for some company websites elsewhere that are in a similar industry or about a similar subject matter. As long as the content is relevant in some way to both websites and holds unique value then it should be worth exploring in my opinion.

No the search engines are unable to distinguish that the pages are a guest post, for them it is just duplicate content.

You could of course try and rewrite your article, but search engines such as Google are getting better and better all the time so are increasingly able to see if you are just re-hashing an article.It's scary!

I guess the take home is to only produce unique content that offers real value to the web user.

Ok great, sounds like we all seem to agree on the fact that syndicating content is not a great idea.

Away from this, is the idea of writing a guest post for them (1 that is clearly introduced as from ourselves) a bad one? Surely this would be a good opportunity to build some links to my website and (being a popular website) drive some traffic as part of the same process.