//All accounts that match the lead emails.
    List<Account> accounts = [SELECT Id 
                              , FirstName
                              , LastName
                              , PersonEmail,
                              Deleted__c
                              FROM Account 
                              WHERE PersonEmail IN: leadEmails];
    
    Map<String, Account> accountsByEmail = new Map<String, Account>();
    
    for(Account account : accounts)
    {
        accountsByEmail.Put(account.PersonEmail, account);
    }
    
    //All the leads that need to be converted into new accounts.
    List<Database.LeadConvert> leadsToConvert = new List<Database.LeadConvert>();
    
    //All the leads that match an existing account. These accounts will be updated and the lead set to converted.
    Set<Id> existingAccountsForLeads = new Set<Id>();
    
    for (Lead lead : newLeadsById.Values())
    {
        //to prevent recursion 
        if (lead.isConverted == false && lead.Email_already_in_the_system__c == false) 
        { 
            Database.LeadConvert leadConvert = new Database.LeadConvert();
            leadConvert.SetLeadId(lead.Id);
            leadConvert.SetDoNotCreateOpportunity(true);
            leadConvert.SetConvertedStatus(convertedLeadStatuses[0].MasterLabel);
            
            //Check whether an account already exists with this lead's email and name. If one already exists we do not need to convert 
            //to a new account as they are already a customer. We rather need to update the existing record.
            Account matchingAccount = accountsByEmail.Get(lead.Email);
            if (matchingAccount != null && matchingAccount.FirstName == lead.FirstName && matchingAccount.LastName == lead.LastName && matchingAccount.Deleted__c == false)
            {
                leadConvert.SetAccountId(matchingAccount.Id);
                existingAccountsForLeads.Add(matchingAccount.Id);
            }
            
            leadsToConvert.Add(leadConvert);
        }
    }