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thanks for the help. I am trying to cover all bases here. Duplicate content was one concern, the other one is too high link density and bad incoming links.

I have downloaded a full backlinks report now using Majestic SEO (OSE only shows incoming links from 74 domains...).
I think I may have found the problem. I used to have a forum on that domain years ago which was hacked and used for a lot of spammy outgoing links for stuff like Cialis, Viagra etc.. Those guys also linked from other sites to my forum pages. Example:

| from: http://www.grupoibira.com/foro/viewto... | Anchor: buy flagyl in lincolnu... | 3 | to: http://www.poker-coaching.net/phpbb3/... |

When I closed the forum and deleted the forum plugin, I redirected all forum pages to my main page which, under the circumstances was a bad mistake I guess. Because with the redirect, all those spammy links now end up pointing to my main page, right? So first, I have removed that redirect now.

But the problem remains that I still have plenty of links from spam sites pointing to URLs of my domain that do not exist any more.
Is there anything else I can do to remove those links or have Google remove/disregard them, or do you think a reconsideration request explaining the situation would help?

Thanks for looking into this. Actually I checked the whole site by doing a batch search on Copyscape and there were only minor duplicate content issues. I resolved those by editing the content parts in question (on February 24th 2012).
Since I am desperately searching for the reasons why this site was penalized (and it def is...), it would be great to know why your duplicate content checker finds errors. Could only be related to multiple versions of one page on different URLs. I do have all http://mysitedotcom redirected to www.mysitedotcom, and the trailing slash/notrailingslash URL problem was also resolved by a redirect long ago, so I do not know where the problem lies.
Thanks for the help!

Yes that is what I first thought too. If only it were that easy.
But when I do, I see a couple of URLs that definitely do not have any duplicate content . Could it be that the dupe content check  considers text in sitewide modules (like the modules "Poker News" and "Tips for ...." in www.poker-coaching.net) as duplicate content, because they appear on all pages?
This way, the duplicate content finding function is totally worthless.

I have a ton of duplicate content warnings for my site poker-coaching.net, but I can't see where there are duplicate URLs. I cannot find any function where I could check the original URL vs a list of other URLs where the duplicate content is?

I just want to make myself a little bit clearer here. While I appreciate any advice on why the site's rankings might be (mildly) affected, I am more interested in finding out why the site might have been penalized (which is most probably the case).
Here are my suspicions:

1. I had three articles that were 45% duplicate - thanks to one of my writers, may he rest in peace ;)), but they were removed/rewritten 2 months ago
2. I had a guy build some links for me about a year ago and he set up a link wheel without my consent, so the link profile might be a problem (too many similar links?)
3. Duplicate content because same article exists on my site with different URLs?
4. Too many links from main page (as suggested by your colleague, although I doubt very much that this can cause a penalty)
5. Panda related - although my content is of good quality
6. Site structure for the multiple languages (.net for german is one Joomla installation, and the other /xlang/ is a separate Joomla install where I use Joomfish for the various translation

Any ideas?

Duplicate URL: but I do have a redirect in place from with / to without, that's why I was surprised to see this pop up as an error? Actually I redirected http://poker-coaching.net, http://poker-coaching.net/ and http://www.poker-coaching.net/;  all point to http://www.poker-coaching.net (suing the function of AceSEF module for this).

Per 1.: Ok this is something I will consider.

Per 2.: I use those redirects on all my sites, never had any problems with those? It's a very common thing with affiliate sites btw.

Per 3.: Isn't it agreed upon that your site cannot be hit with a penalty because a bad neighborhood site links to you (as opposed to me linking to such a site, which of course can be a big problem)? Webmasters don't have control over sites that give them links. If Google were to penalize me for that, would we all be google-bowling our competitors sites?
I can see problems with a link profile where maybe too many links have the same title and thus look unnatural, when you look at my link profile would you say that might be the case?

This site has given me so much problem I was considering "selling" it to someone else, host/rebuild it on a different server and do a reconsideration request telling Google I am the new owner... Tried regular reconsideration requests several times, but G always tell me there is no manual penalty, so I must have tripped some algorithm. Question is, which one?

Thanks a lot!

My crawl report for www.poker-coaching.net show a number of duplicate content warnings.
First, it shows a warning for www.poker-coaching.net and www.poker-coaching.net/, although there is a redirect in place for this, and for non-www. to www. as well. So IMO there is no duplicate content here, so why the error message?
Next, it shows a duplicate content warning for a number of pages, but when I click the number on the right, there is no info an which pages contain the duplicate content. Will this info be there later when the crawl is completed?

My problem is that this is my oldest site, but since over one year it seems to have a penalty which I can't get rid of - I cannot identify the reason for the penalty. I already rebuilt the site from scratch once, but I still do not see results.

If a membership at SEOMOZ would be able to help me bring this site back on track, that would already be worth one years membership fees, so I'd really be very thankful if someone could take a look at the site and give me an idea about what needs to be done here.

My site www.poker-coaching.net is built with Joomla, and I have one main menu that links all articles (app. 120) in one language from the start page. The site has a language selection menu with translations available in 4 other languages, and for each language, there are another 80 articles linked from the main page.

The SEOMOZ crawling report now say that I have too many links from the main page.

I always thought it was a good thing for SEO to have articles linked from the main page - do I need to restructure my site and start building multi-level menu structures, where I would have a main menu on the start page and the next level menu would only be shown after a user clicks on a main menu item?