Show alert message in jquery

Question

I want show a alert message when list size is greater than zero. I received like this error.

This is my controller:

public class OpportunityVFPageExtension {

    public Opportunity getOpportunity {get;set;}
    public List<Opportunity> oldOpportunity {get;set;}

    public OpportunityVFPageExtension(ApexPages.StandardController sc){
        getOpportunity = (Opportunity)sc.getRecord();
    }

    Public Void Save(){
        oldOpportunity = [SELECT Id,Name FROM Opportunity WHERE Name =: getOpportunity.Name]; 
        insert getOpportunity;
    }
}

This is my VF page:

<apex:page standardController="Opportunity" extensions="OpportunityVFPageExtension">
    <apex:sectionHeader title="New Opportunity"/>
    <script>
    function validateName(){
           if('{oldOpportunity.size}' > 0){
            alert('This is duplicate record. Do you want to submit?');
            }
        }
    </script> 
    <apex:form >
            <!--    <apex:actionFunction name="callSave" action="{!Save}"/>-->
        <apex:pageBlock title="Opportunity Edit" >
            <apex:pageBlockButtons >
                <apex:commandButton value="Save" action="{!Save}" onclick="validateName();" />
                <apex:commandButton value="Cancel" action="{!Cancel}" />
            </apex:pageBlockButtons>
            <apex:pageBlockSection title="Opportunity Information"  columns="1" collapsible="true" >
                <apex:inputField value="{!Opportunity.Name}" />
                <apex:inputField value="{!Opportunity.CloseDate}" />
                <apex:inputField value="{!Opportunity.StageName}" />
                <apex:inputField value="{!Opportunity.Last_Name__c}" />
                <apex:inputField value="{!Opportunity.Email__c}" />
                <apex:inputField value="{!Opportunity.Phone__c}" />
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>

Thanks in advance.

Answer 1

To find duplicate and insert new record only if duplicate does not exist modify your save method.

Public pagereference Save(){
  oldOpportunity = [SELECT Id,Name FROM Opportunity WHERE Name =: getOpportunity.Name]; 
  if(oldOpportunity.isEmpty()){
        insert getOpportunity;
  }
  return null
}