public class ApplyDiscount{

public static void insertdiscount(List<opportunity> opplist){
    for(Opportunity oa: opplist){
      if(oa.IsDiscountApplied__c != True)
     { oa.Amount= oa.Amount - oa.Amount*(0.1);
       oa.IsDiscountApplied__c=True;
      }          
   }
    }
}