Item (Mage_SalesRule_Model_Rule) with the same id “5” already exist

Question

I am trying to debug this error which happens whenever a logged in customer adds a coupon code during checkout.

a:5:{i:0;s:67:"Item (Mage_SalesRule_Model_Rule) with the same id "5" already exist";i:1;s:1977:"#0 /var/www/vhosts/test.devcom/httpdocs/lib/Varien/Data/Collection/Db.php(576): Varien_Data_Collection->addItem(Object(Mage_SalesRule_Model_Rule))
#1 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/SalesRule/Model/Validator.php(117): Varien_Data_Collection_Db->load()
#2 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/SalesRule/Model/Quote/Freeshipping.php(60): Mage_SalesRule_Model_Validator->init('1', '1', 'INSTA20')
#3 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Sales/Model/Quote/Address.php(1013): Mage_SalesRule_Model_Quote_Freeshipping->collect(Object(Mage_Sales_Model_Quote_Address))
#4 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Sales/Model/Quote.php(1331): Mage_Sales_Model_Quote_Address->collectTotals()
#5 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Checkout/Model/Cart.php(458): Mage_Sales_Model_Quote->collectTotals()
#6 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Checkout/controllers/CartController.php(127): Mage_Checkout_Model_Cart->save()
#7 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Checkout_CartController->indexAction()
#8 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('index')
#9 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#10 /var/www/vhosts/test.dev.com/httpdocs/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#11 /var/www/vhosts/test.dev.com/httpdocs/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#12 /var/www/vhosts/test.dev.com/httpdocs/index.php(87): Mage::run('', 'store')
#13 {main}";s:3:"url";s:15:"/checkout/cart/";s:11:"script_name";s:10:"/index.php";s:4:"skin";s:7:"default";}

I'm not sure what is causing the error since the coupon code works when checking out as a guest.

Has anyone had this error before and know what the cause could be? I'd appreciate any assistance please...