trigger emiTrigger on EMI_Payment__c (after insert) {
      Map<ID, Loans__c> parentLoans = new Map<ID, Loans__c>(); 
      List<id> listIds = new List<id>();
      Double Principal_Outstanding_at_the_end_of_the_month;
            
      for(EMI_Payment__c e : Trigger.new){
      listIds.add(e.Loan_Id__c);//getting EMI ID(s)
      }
   //getting parent record ID and data with above ID
   parentLoans = new Map<Id,Loans__c>([SELECT id,paid_amount__c,Loan_Amount__c,TenureYrs__c,
   Interest_Rate__c,Outstanding_Principal__c,(SELECT id,amount__c from
   EMI_Payment__r) from Loans__c  where ID IN : listIds ]);  
   //updating paid amount with EMI 
   for(EMI_Payment__c e:Trigger.new){
         Loans__c myloan = parentLoans.get(e.Loan_Id__c);
         
        Double  principal = myloan.Loan_Amount__c;
        Decimal tenure = myloan.TenureYrs__c*12;
        Decimal interest = myloan.Interest_Rate__c/12/100;
        Decimal pr = 1 + interest;
        Double EMI =  principal * interest * Math.pow(pr.doubleValue(),tenure.doubleValue())/(Math.pow(pr.doubleValue(),tenure.doubleValue())-1);
      
            Double Interest_Component_of_EMI = interest * myloan.Outstanding_Principal__c;
            Double Principal_Component_of_EMI = Math.round(EMI - Interest_Component_of_EMI);
            Principal_Outstanding_at_the_end_of_the_month = myloan.Outstanding_Principal__c - Principal_Component_of_EMI;
        
            myloan.Outstanding_Principal__c-= Principal_Component_of_EMI;//decrementing outstading balance
            myloan.paid_amount__c += e.amount__c; //incrementing paid amount
   }
   
   if(parentLoans.size()>0){
   upsert parentLoans.values();  // error occurring here
   }
}

trigger emiTrigger on EMI_Payment__c (after insert) {
Set<id> loanid = new Set<id>();
List<Loans__c> loanslist = new List<Loans__c>();
for(EMI_Payment__c e : Trigger.new){
   loanid.add(e.Loan_Id__c);
}
for(Loans__c loan : [select id,Name,EMI__c,paid_amount__c,(select id,amount__c from EMI_Payment__r) from Loans__c where id IN : loanid]){
   for(EMI_Payment__c e:loan.EMI_Payment__r){
         //ading Emi amount to old paid amount , its giving worng value
         loan.paid_amount__c = loan.paid_amount__c +e.amount__c;
         system.debug('updated for...'+loan.paid_amount__c+'...>'+ e.amount__c);
   }
   loanslist.add(loan);
}
update loanslist;
system.debug('updated ...');
}

==================================================
Loans__c  ==> parent object
EMI_Payment__c  ==> child object
trying to update   loan.paid_amount__c with new EMI payment amount but its coming up with wrong value but if I debug for same value its showing correct in logs

Example : Assume 45281 is a initial value of loan.paid_amount__c 
and next if I give 45281 as input  for e.amount__c
expected to happen -
loan.paid_amount__c = loan.paid_amount__c +e.amount__c;
45281 = 45281 + 45281; ==> 96502
but its coming up with ==> 144,753

I am not understanding why ?