Trigger to avoid duplicate records on insert/update

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trigger FindDuplicate on Account (before insert, before update) { set<string> NameSet = New set<string>(); for(Account onn : Trigger.new){ NameSet.add(onn.Name); } if(NameSet.size()>0 && NameSet.size()!=null){ List<Account > NameList = [Select id, Name from Account WHERE Name in: Nameset]; Map<string,Account > NameCompareMap = new Map<String,Account >(); for(Account One : NameList){ NameCompareMap.put(one.Name, one); } for(Account On1 : trigger.new){ if(NameCompareMap.containsKey(on1.Name)){ On1.Name.adderror('NaMe AlReAdY eXiStS'); } } } }