Hi All, 
          I am getting this error in Class: LeadTrigger: execution of AfterInsert caused by: System.ListException: Duplicate id in list: 00Q8I000006zCO1UAM Class.LeadTriggerHelper.markPrimary: line 168, column 1 Trigger.LeadTrigger: line 14, column 1
My class is given below:- 
public class LeadTriggerHelper {
    
    public static List<Lead> newLeads = new List<Lead>();
    public static List<Lead> oldLeads = new List<Lead>();
    public static Map<Id, Lead> newMapLeads = new Map<Id, Lead>();
    public static Map<Id, Lead> oldMapLeads = new Map<Id, Lead>();
    public static Boolean runTrigger = true;
    private static Map<String, Schema.SObjectField> leadFieldsMap = Lead.SObjectType.getDescribe().fields.getMap();
    
    
    public static void markPrimary(){
        
        Map<String, List<Lead>> mapKeyToAssociatedLeads = new Map<String, List<Lead>>();
    List<String> fields = getFieldsString(); 
    List<Lead> applicableLeads = (Lead[]) Database.query('SELECT ' + String.join(fields, ',') +
                                                        ' FROM Lead WHERE Id IN :newLeads');

    Set<String> leadEmails = new Set<String>();
    Set<String> nameAndPhoneKeys = new Set<String>();
    Set<String> phones = new Set<String>();
    Set<String> names = new Set<String>();

    List<Lead> leadsToUpdate = new List<Lead>();
    for (Lead leadObj : applicableLeads) {
        if (leadObj.Email != null) {
            leadEmails.add(leadObj.Email);
        }
        names.add(leadObj.Name);
        phones.add(leadObj.Phone);
        nameAndPhoneKeys.add(leadObj.Name + leadObj.Phone);
    }

    String leadQuery = 'SELECT ' + String.join(fields, ',') +
                       ' FROM Lead WHERE (Email IN :leadEmails OR (Name IN :names AND Phone IN :phones))' +
                       ' AND Id NOT IN :applicableLeads AND IsConverted = False AND Lead_Type__c != \'Primary Converted\'' +
                       ' ORDER BY CreatedDate ASC';

    String personAccountQuery = 'SELECT Id, PersonEmail, PersonMobilePhone, Full_Name__pc ' +
                                ' FROM Account WHERE (PersonEmail IN :leadEmails OR ' +
                                ' (Full_Name__pc IN :names AND PersonMobilePhone IN :phones))';
        
        System.debug('personAccountQuery'+personAccountQuery);

    List<SObject> combinedResults = new List<SObject>();
    combinedResults.addAll(Database.query(leadQuery));
    combinedResults.addAll(Database.query(personAccountQuery));
        System.debug('combinedResults'+combinedResults);

    for (SObject existingRecord : combinedResults) {
        if (existingRecord.getSObjectType() == Lead.SObjectType) {
            Lead existingLead = (Lead) existingRecord;

            if (leadEmails.contains(existingLead.Email)) {
                if(mapKeyToAssociatedLeads.containsKey(existingLead.Email)) {
                List<Lead> associatedLeads = mapKeyToAssociatedLeads.get(existingLead.Email);
                associatedLeads.add(existingLead);
                mapKeyToAssociatedLeads.put(existingLead.Email, associatedLeads);
            }  
            else {
                mapKeyToAssociatedLeads.put(existingLead.Email, new List<Lead> {existingLead});
            }
            }

            if (nameAndPhoneKeys.contains(existingLead.Name + existingLead.Phone)) {
                if(mapKeyToAssociatedLeads.containsKey(existingLead.Name+existingLead.Phone)) {
                List<Lead> associatedLeads = mapKeyToAssociatedLeads.get(existingLead.Name+existingLead.Phone);
                associatedLeads.add(existingLead);
                mapKeyToAssociatedLeads.put(existingLead.Name+existingLead.Phone, associatedLeads);
            }  
            else {
                mapKeyToAssociatedLeads.put(existingLead.Name + existingLead.Phone, new List<Lead> {existingLead});
            }
            }

            // Check if it's a matching person account
         } else if (existingRecord.getSObjectType() == Account.SObjectType) {
             Account matchingPersonAccount = (Account) existingRecord;
               System.debug('matchingPersonAccount'+matchingPersonAccount);
            
         String personAccountKey = matchingPersonAccount.PersonEmail != null ? matchingPersonAccount.PersonEmail : (matchingPersonAccount.Full_Name__pc + matchingPersonAccount.PersonMobilePhone);
        if (!mapKeyToAssociatedLeads.containsKey(personAccountKey)) {
            mapKeyToAssociatedLeads.put(personAccountKey, new List<Lead>());
        }
             System.debug('mapKeyToAssociatedLeads'+mapKeyToAssociatedLeads);

        for (Lead leadObj : applicableLeads) {
            if (leadObj.Email != null && leadObj.Email == matchingPersonAccount.PersonEmail) {
                mapKeyToAssociatedLeads.get(personAccountKey).add(leadObj);
                
            } else if (leadObj.Name != null && leadObj.Phone != null && (leadObj.Name + leadObj.Phone) == (matchingPersonAccount.Full_Name__pc + matchingPersonAccount.PersonMobilePhone)) {
                mapKeyToAssociatedLeads.get(personAccountKey).add(leadObj);
            }
        }
            
            System.debug('mapKeyToAssociatedLeads Line 94'+mapKeyToAssociatedLeads);
            
             for (Lead leadObj : applicableLeads) {
                System.debug('mapKeyToAssociatedLeads'+mapKeyToAssociatedLeads);
                if (mapKeyToAssociatedLeads.containsKey(leadObj.Email) && mapKeyToAssociatedLeads.get(leadObj.Email).contains(leadObj)) {
                    leadObj.Person_Account__c = matchingPersonAccount.Id;
                    System.debug('leadObj.Person_Account__c'+leadObj.Person_Account__c);
                } else if (mapKeyToAssociatedLeads.containsKey(leadObj.Name + leadObj.Phone) && mapKeyToAssociatedLeads.get(leadObj.Name + leadObj.Phone).contains(leadObj)) {
                    leadObj.Person_Account__c = matchingPersonAccount.Id;
                    System.debug('leadObj.Person_Account__c'+leadObj.Person_Account__c);
                }
            }
        }
    }
        
        List<Lead> associatedLeads = new List<Lead>();
        Map<Id, Lead> mapNewLeadToAssociatedLead = new Map<Id, Lead>();
        for(Lead leadObj : applicableLeads){
            
            if(mapKeyToAssociatedLeads.containsKey(leadObj.Email)){
                associatedLeads = mapKeyToAssociatedLeads.get(leadObj.Email);
                mapNewLeadToAssociatedLead.put( leadObj.Id, associatedLeads.get(0) );
            }
            else if(mapKeyToAssociatedLeads.containsKey(leadObj.Name+leadObj.Phone)){
                associatedLeads = mapKeyToAssociatedLeads.get(leadObj.Name+leadObj.Phone);
                mapNewLeadToAssociatedLead.put( leadObj.Id, associatedLeads.get(0) );
            }
        }
        
        System.debug('mapNewLeadToAssociatedLead'+mapNewLeadToAssociatedLead);
        
        // loop over new inserted leads
        for(Lead leadObj : applicableLeads){
            
            Lead primaryLead;
            if( mapNewLeadToAssociatedLead.containsKey( leadObj.Id )) {
                primaryLead = mapNewLeadToAssociatedLead.get( leadObj.Id );
            }
            System.debug('primaryLead'+primaryLead);
            if( primaryLead != null ) {
                primaryLead.Lead_Type__c = 'Primary';
                // update new lead to existing type        
                leadObj.Lead_Type__c = 'Existing Lead';
                leadObj.Primary_Lead__c = primaryLead.Id;
                //leadObj.OwnerId = primaryLead.OwnerId;
                runTrigger = false;
                leadsToUpdate.add(leadObj);
                System.debug('leadsToUpdate'+leadsToUpdate);
                Schema.SObjectType leadType = leadObj.getSObjectType();
                
                for(String fieldName : leadFieldsMap.keySet()){
                    if(fieldName != 'Primary_Lead__c'){
                        if(leadFieldsMap.get(fieldName).getDescribe().isUpdateable()){
                            Object associatedLeadFieldValue = leadObj.get(fieldName);
                            Object primaryLeadFieldValue = primaryLead.get(fieldName);
                            
                            if (primaryLeadFieldValue == null && associatedLeadFieldValue != null) {
                                primaryLead.put(fieldName, associatedLeadFieldValue);
                            }    
                        }
                    }
                }
                leadsToUpdate.add(primaryLead); 
                 System.debug('primaryLead'+primaryLead);
            }
            else {
                leadObj.Lead_Type__c = 'Primary';
                leadsToUpdate.add(leadObj);
                System.debug('leadObj'+leadObj.Lead_Type__c);
            }
        }
        if(leadsToUpdate.size() > 0){
              System.debug('leadsToUpdate'+leadsToUpdate);
             update leadsToUpdate; // Getting error here
         }
    }
         }
Can anyone help me to remove this error from this class?