Magento Error : "You cannot define a correlation name 'size_idx' more than once"....

>> The error occurs if you use <block type="catalog/layer_view" name="catalog.leftnav" after="currency" template="catalog/layer/view.phtml"/> this block twice in your catalog.xml(in your theme folder).
Make sure there is only one layered block.

I got this error when I copied the leftnav block within content block without removing the leftnav block from the left block.

<reference name="left">
            <block type="catalog/layer_view" name="catalog.leftnav" after="currency" template="catalog/layer/view.phtml"/>
</reference>
<reference name="content">
            <block type="catalog/category_view" name="category.products" template="catalog/category/view.phtml">
                <block type="catalog/layer_view" name="content.leftnav" as="cat_left_nav" template="catalog/layer/view.phtml"/>
</reference>