My requirement is to take any pricebook entry made in the standard pricebook and replicate/update it in a second price book. With the code below I keep getting  the error below. I have read everything I could find, but I cannot figure out what I need to change.

FIELD_INTEGRITY_EXCEPTION, This price definition already exists in this price book: []

Also, I need the logic to look and see if the pricebookentry (product2Id) already exists and if so, just update the unitprice. But anytime I add that logic I get too many SOQL queries. (I was adding a query to look up the pricebookentries in the second price book, with an if statement to see if the product2id already existed. I had it in the for loop of the code below. - so 2 for loops and an if statement. It was the only way I could figure out how to get the logic, and obviously it was not correct.) Any help would be much appreciated.

global class pricebookEntryCopybatch implements Database.Batchable<sObject>{
        public final string query;
        Set<Id> pbeIds = new Set<Id>();
        Pricebook2 stdPrice = [Select id, isActive from Pricebook2 where isStandard=true limit 1];
        Pricebook2 fdaPrice = [Select id, isActive  from Pricebook2 where Name='FDA Pricebook' limit 1];    
    global pricebookEntryCopybatch(){
      query = 'Select Id, Pricebook2Id, isActive, SystemModStamp from PriceBookEntry ';
  }
        global Database.querylocator start(Database.BatchableContext BC){
                return Database.getQueryLocator(query);
        }


    global void execute(Database.BatchableContext BC, List<sObject> scope){
        for(sObject s : scope){
        for(PricebookEntry pbe : (List<PricebookEntry>)scope){
            if(pbe.Pricebook2Id == stdPrice.Id && pbe.IsActive == true && pbe.SystemModStamp > Datetime.now().addMinutes(-15)){
               pbeIds.add(pbe.id); 
            }
        }
      
      List<PricebookEntry> pbeforinsert = new List<PricebookEntry>();
            system.debug('***************************************stdPrice ' +stdPrice);
            system.debug('***************************************fdaPrice ' +fdaPrice);
      List<PricebookEntry> stdPBE = [SELECT Pricebook2Id, IsActive, UnitPrice, Id, Product2Id, CurrencyIsoCode FROM PricebookEntry Where Id in: pbeIds];
        for(PricebookEntry pbecopy : stdPBE){
            PricebookEntry pbesToAdd = new PricebookEntry();{
                pbesToAdd.Pricebook2Id = fdaPrice.Id;
                pbesToAdd.IsActive = pbecopy.IsActive;
                pbesToAdd.Product2Id = pbecopy.Product2Id;
                pbesToAdd.UnitPrice = pbecopy.UnitPrice;
                pbesToAdd.CurrencyIsoCode = pbecopy.CurrencyIsoCode;
                pbeforinsert.add(pbesToAdd);
                
            }
        }
          insert pbeforinsert;  
        }
    }
        
        
      
            global void finish(Database.BatchableContext BC){
   }   


}