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
or a file from your theme
and remove block with navigation:
<block type="catalog/layer_view" ...... />
Then refresh cache.