Remove redundant line

Author: edenduong

app/code/Magento/Sales/i18n/en_US.csv

@@ -799,5 +799,4 @@ Refunds,Refunds
 "Allow Zero GrandTotal","Allow Zero GrandTotal"
 "Could not save the shipment tracking","Could not save the shipment tracking"
 "Please enter a coupon code!","Please enter a coupon code!"
-"Please enter a coupon code!","Please enter a coupon code!"
 "Reorder is not available.","Reorder is not available."