Duplicate Content - Just how killer is it?

[Ask_MMM]

Yesterday I received my ranking report and was extremely disappointed that my high-priority pages dropped in rank for a second week in a row for my targeted keywords.  This is after running them through the gradecard and getting As for each of them on the keywords I wanted.

I looked at my google webmaster tools and saw new duplicate content pages listed, which were the ones I had just modified to get my keyword targeting better.  In my hastiness to work on getting the keyword usage up, I neglected to prevent these descriptions from coming up when viewing the page with filter parameters, sort parameters and page parameters... so google saw these descriptions as duplicate content (since myurl.html and myurl.html?filter=blah are seen as different).

So my question: is this the likely culprit for some pretty drastic hits to ranking?  I've fixed this now, but are there any ways to prevent this in the future? (I know _of _canonical tags, but have never used them, and am not sure if this applies in this situation)

Thanks!

EDIT: One thing I forgot to ask as well: has anyone inflicted this upon themselves? And how long did it take you to recover?

[SEOTGT]

Duplicate content can pack a double punch since not only does it potentially look like spam to Google but it also divides up the value of the pages between the multiple sources. Best case is to eliminate duplicate content, but often the easier fix--especially when parameters are involved--is to use canonical.

Google doesn't always seem to respect the canonical tag, but it should help mitigate duplicate content errors that can definitely drag down the rankings.

[bronxpad]

Just a note. Besides the canonical fix, you can also go into google webmasters and instruct them if you want them to show the www version or non www version. This can fix the url side of duplicate content. This doesn't fix the whole problem but is a good feature to utilize. I hope this helps.

[mytouchoftech]

The canonical tags are definitely applicable to this situation once Google goes everything again and sorts it all out with the new tags.