Salesforce trigger could allow one parent will have only one child and same parent doesn't allow duplicates

Trigger Code :
----------------
trigger restrictChildDuplicates on Contact (before Insert) {
    if(System.trigger.isInsert && System.trigger.isBefore){
        restrictContactDuplicateHelper.avoidDuplicateRecords(trigger.new);
    }
}

Helper Apex Class :
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    public class restrictContactDuplicateHelper{
    public restrictContactDuplicateHelper(){
 
    }
    public static set<Id> accId = new set<Id>();
    public static map<Id, Set<Decimal>> accConMap = new map<Id, Set<Decimal>>();
    public static void avoidDuplicateRecords(List<Contact> conLis){
        for(Contact con : conLis){
            if(con.AccountId != null){
                accConMap.put(con.AccountId, new set<Decimal>());
            }
       }
       if(!accConMap.isEmpty()){
            for(Contact con : [Select Id,Priority__c,AccountId from Contact where AccountId =: accConMap.keySet()]){
                accConMap.get(con.AccountId).add(con.Priority__c);
            }
      }
      if(!accConMap.isEmpty()){
        for(Contact conRec : conLis){
             if(accConMap.containsKey(conRec.AccountId)){              
                 Set<Decimal> conSet = accConMap.get(conRec.AccountId);
                 if(conSet.contains(conRec.Priority__c)){
                     conRec.Priority__c.addError('Priority was registered already for the Account.');
                }
            }
         }
      }
    }
}

Show or Hide fields in visualforce page using action function

VF:
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<apex:page controller="actionFunctionController" tabStyle="Account">

    <apex:form >

        <apex:actionFunction name="priorityChangedJavaScript" action="{!priorityChanged}" rerender="out"/>

        <apex:pageBlock >

            <apex:pageBlockSection title="If you will select High Customer Priority then phone textbox will be shown" columns="1" id="out" collapsible="false">

                <apex:inputField value="{!acc.CustomerPriority__c}" onchange="priorityChangedJavaScript()"/>

                <apex:inputField value="{!acc.Phone}" rendered="{!showPhone}"/>

            </apex:pageBlockSection>

        </apex:pageBlock>

    </apex:form>

</apex:page>




Apex:

-----

public class actionFunctionController {

    public Account acc{get;set;}

    public Boolean showPhone{get;set;}

 

    public actionFunctionController(){

        acc = new Account();

        showPhone = false;

    }

 

    public PageReference priorityChanged(){

        if(acc.CustomerPriority__c == 'High'){

            showPhone = true;

        }

        else{

            showPhone = false;

        }

        return null;

    }

}

Screenshot:

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