You cannot define a correlation name 'course_start_date_t1' more than once

#0 /home/qursch/public_html/lib/Varien/Db/Select.php(281): Zend_Db_Select->_join('left join', Array, 'e.entity_id=cou...', Array, NULL)
#1 /home/qursch/public_html/lib/Zend/Db/Select.php(357): Varien_Db_Select->_join('left join', Array, 'e.entity_id=cou...', Array, NULL)
#2 /home/qursch/public_html/app/code/core/Mage/Eav/Model/Entity/Attribute/Source/Table.php(122): Zend_Db_Select->joinLeft(Array, 'e.entity_id=cou...', Array)
#3 /home/qursch/public_html/app/code/core/Mage/Catalog/Model/Resource/Product/Collection.php(1593): Mage_Eav_Model_Entity_Attribute_Source_Table->addValueSortToCollection(Object(Ecommage_Rewriteajaxifiedproductedit_Model_Catalog_Resource_Product_Collection), 'ASC')
#4 /home/qursch/public_html/app/code/core/Mage/Eav/Model/Entity/Collection/Abstract.php(1442): Mage_Catalog_Model_Resource_Product_Collection->addAttributeToSort('course_start_da...', 'ASC')
#5 /home/qursch/public_html/lib/Varien/Data/Collection/Db.php(649): Mage_Eav_Model_Entity_Collection_Abstract->_renderOrders()
#6 /home/qursch/public_html/app/code/local/Ecommage/Locator/Helper/Data.php(241): Varien_Data_Collection_Db->getData()
#7 /home/qursch/public_html/app/design/frontend/clean/qurs/template/locator/product/list.phtml(105): Ecommage_Locator_Helper_Data->setDataPositions(Object(Ecommage_Rewriteajaxifiedproductedit_Model_Catalog_Resource_Product_Collection), Array)
#8 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(241): include('/home/qursch/pu...')
#9 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(272): Mage_Core_Block_Template->fetchView('frontend/clean/...')
#10 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(286): Mage_Core_Block_Template->renderView()
#11 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(923): Mage_Core_Block_Template->_toHtml()
#12 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(641): Mage_Core_Block_Abstract->toHtml()
#13 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(585): Mage_Core_Block_Abstract->_getChildHtml('product_list', true)
#14 /home/qursch/public_html/app/code/core/Mage/Catalog/Block/Category/View.php(90): Mage_Core_Block_Abstract->getChildHtml('product_list')
#15 /home/qursch/public_html/app/design/frontend/clean/default/template/catalog/category/view.phtml(76): Mage_Catalog_Block_Category_View->getProductListHtml()
#16 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(241): include('/home/qursch/pu...')
#17 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(272): Mage_Core_Block_Template->fetchView('frontend/clean/...')
#18 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(286): Mage_Core_Block_Template->renderView()
#19 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(923): Mage_Core_Block_Template->_toHtml()
#20 /home/qursch/public_html/app/code/core/Mage/Core/Block/Text/List.php(43): Mage_Core_Block_Abstract->toHtml()
#21 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(923): Mage_Core_Block_Text_List->_toHtml()
#22 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(641): Mage_Core_Block_Abstract->toHtml()
#23 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(585): Mage_Core_Block_Abstract->_getChildHtml('content', true)
#24 /home/qursch/public_html/app/design/frontend/clean/qurs/template/page/1column.phtml(102): Mage_Core_Block_Abstract->getChildHtml('content')
#25 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(241): include('/home/qursch/pu...')
#26 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(272): Mage_Core_Block_Template->fetchView('frontend/clean/...')
#27 /home/qursch/public_html/app/code/core/Mage/Core/Block/Template.php(286): Mage_Core_Block_Template->renderView()
#28 /home/qursch/public_html/app/code/core/Mage/Core/Block/Abstract.php(923): Mage_Core_Block_Template->_toHtml()
#29 /home/qursch/public_html/app/code/local/Inchoo/PHP7/Model/Layout.php(15): Mage_Core_Block_Abstract->toHtml()
#30 /home/qursch/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(390): Inchoo_PHP7_Model_Layout->getOutput()
#31 /home/qursch/public_html/app/code/core/Mage/Catalog/controllers/CategoryController.php(161): Mage_Core_Controller_Varien_Action->renderLayout()
#32 /home/qursch/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Catalog_CategoryController->viewAction()
#33 /home/qursch/public_html/app/code/local/Mage/Core/Controller/Varien/Router/Standard.php(255): Mage_Core_Controller_Varien_Action->dispatch('view')
#34 /home/qursch/public_html/app/code/local/SM/Vendors/Controller/Router/Vendor.php(177): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#35 /home/qursch/public_html/app/code/local/Ecommage/Rewritevendor/Controller/Router/Vendor.php(292): SM_Vendors_Controller_Router_Vendor->match(Object(Mage_Core_Controller_Request_Http))
#36 /home/qursch/public_html/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Ecommage_Rewritevendor_Controller_Router_Vendor->match(Object(Mage_Core_Controller_Request_Http))
#37 /home/qursch/public_html/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#38 /home/qursch/public_html/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#39 /home/qursch/public_html/index.php(88): Mage::run('', 'store')
#40 {main}