trigger ownerupdate on Account (before insert) {
set<id> setAccOwners= new set<id>();

for(Account acc: Trigger.new){
setAccOwners.add(acc.ownerID);
}

Map<id,user> user_map = new map<id,user>([select name from user where id in : setAccOwners]);
for (Account acc: Trigger.new){
user u = user_map.get(acc.ownerID);
 acc.RecordOwner__c = u.name;
}
}