You cannot define a correlation name '*_idx' more than once

Not needed since Improved navigation 2.5.1. version.

If you are using Improved Layered Navigation and get this message, it means that the category page has two blocks of the navigation in the left column. Please open your catalog layout

app\design\frontend\default\default\layout\catalog.xml

or a file from your theme

app\design\frontend\<YOUR-PACKAGE>\<YOUR-THEME>\layout\catalog.xml

and remove block with navigation:

<reference name="left">
 <block type="catalog/layer_view" ......  /> 
 </reference>

Then refresh cache.