public pageReference save(){
    try{
        upsert cityNames;
    }catch(Exception e){
        String error = e.getMessage();
        ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,error));
    }

    return new pageReference('/apex/CityControllerPage').setRedirect(true);

}

if(Trigger.isInsert){
        for(City_Name__c  ct : Trigger.New){
             if(ct.name!=null && cList.size()>0){
                 ct.name.addError('City Name already Exits');
             }
        }
    }

<apex:page controller="Ciler" tabStyle="Contact">

<apex:pagemessages ></apex:pagemessages>
<apex:form >
    <apex:pageBlock >
        <apex:pageBlockButtons location="top" >
            <apex:commandButton action="{!save}" value="Save"/>
        </apex:pageBlockButtons>

        <apex:pageBlockSection title="City Input">
            <apex:inputField value="{!cityNames.name}"/>
            <apex:inputField value="{!cityNames.Pin__c}"/>
        </apex:pageBlockSection>
    </apex:pageBlock>