Item (mage_salesrule_model_rule) con la misma identificación "5" ya existe

Question

Estoy tratando de depurar este error que ocurre cada vez que un cliente inicio de sesión agrega un código de cupón durante el pago.

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";}

No estoy seguro de qué está causando el error, ya que el código de cupón funciona al verificar como invitado.

¿Alguien ha tenido este error antes y sabe cuál podría ser la causa? Agradecería cualquier ayuda por favor ...