Yesterday I noticed that my alexa rank fell about 18% so I checked a few longtail keywords (which I know I rank on page 1 for) and noticed that I'm not showing in serps for them anymore. I then had a look in WMT and noticed that indexed URLs from my sitemap dropped from over 700 down to 546 (with 1248 submitted).
I then checked duplicate meta descriptions and found a description with over 602 duplicates. These duplicates were a particular category with all the different parameters. ie category-name/?cat=138&dir=asc&limit=10&order=position&p=1&price=1%2C100.
What effect & crawl setting should I be using for the "cat" parameter and could these duplicates have caused my sudden drop?