I am working with a trigger, apex and a visualforce page,
trying to display in my vfpage the error of duplicate values, and it works kind of.
The error message appears in my visual, but it is giving me all the records created in my Factura__c
and not the one of duplicate value. I think is because my trigger it not working well
Example:

In the top of my visual is showing me 3 errors and that is because there 3 records created in my Factura__c
and the only one that it should be displaying it is in the red box.

My object is Factura__c and the custom field that it should not have duplicate values is called Pre_impreso_001_001__c




my trigger

trigger MensajeValorDuplicadoCentro on Factura__c (before insert, before update) {

    Map<Decimal, Factura__c> facMap = new Map<Decimal, Factura__c>();

    For (Factura__c factura : System.Trigger.new) {     
        If ((factura.Pre_impreso_001_001__c != null) && (System.Trigger.isInsert ||
        (factura.Pre_impreso_001_001__c != System.Trigger.oldMap.get(factura.Id).Pre_impreso_001_001__c))){            
            facMap.put(factura.Pre_impreso_001_001__c, factura);                
        }                  
    }
  

    For (Factura__c factura : [SELECT Id, Name, Sucursal__c FROM Factura__c]) {  		
        If (factura.Sucursal__c=='Maker Centro'){      	
            For (Factura__c facturace : [SELECT Pre_impreso_001_001__c FROM Factura__c WHERE Pre_impreso_001_001__c IN :facMap.KeySet()]) {                                                                
                Factura__c newFactura = facMap.get(facturace.Pre_impreso_001_001__c);                  
                newFactura.addError('El #Pre-Impreso(001-001) ingresado ya fue utilizado en la Factura:<a href=\'https://cs21.salesforce.com/'+factura.Id+'\'>'+factura.Name+'</a>', false);           
            }       	
        } 
    } 

}

my visual

<apex:page controller="FacturaRapidaController" tabStyle="Factura__c">  
    <script>     
    function confirmarCancelar() {           
        var isCancel = confirm("Estas seguro que desea cancelar la creación de Factura?");        
        if (isCancel)return true;    
        return false;
    } 
    </script>     
    <apex:sectionHeader title="Creación de" subtitle="FACTURA"/>                  
    <apex:form >      
        <apex:pageBlock mode="edit" title="Crear Factura Contado">                
                         
          	<div style="font-size: 12px; color:#FF0000;">                                                                                              
        		<apex:pageMessages id="errMessages" escape="false"/>                                                                  
    		</div>    
            <apex:pageBlockButtons >                          
                <apex:commandButton action="{!guardarFactura}" value="Guardar"/>                                
                <apex:commandButton action="{!cancelarFactura}" value="Cancelar" onclick="return confirmarCancelar()" immediate="true"/>                      
            </apex:pageBlockButtons> 
            
            <apex:pageBlockSection title="Información Básica de la Factura" columns="1">                                     
                <apex:inputField value="{!factura.Pre_Factura__c}"/>                 
                <apex:inputField value="{!factura.Sucursal__c}" required="true"/>                
                <apex:inputField value="{!factura.Caja__c}" required="true"/>                        
                <apex:inputField value="{!factura.Serie__c}" required="true"/>                
            </apex:pageBlockSection>   
                                 
            <apex:pageBlockSection title="Confirmar Factura" columns="1">            
                <apex:inputField value="{!factura.Pre_impreso_001_001__c}"/>                                                                                                                  
            </apex:pageBlockSection>                                      
        </apex:pageBlock>       
    </apex:form>
</apex:page>

my apex

public PageReference guardarFactura(){                  
        try{ 
            insert factura; 
        }
        catch(exception ex) {  
            ApexPages.addMessages(ex);
            return null;
        }

        PageReference FacturaRapidaPage = new ApexPages.StandardController(factura).view();
        FacturaRapidaPage.setRedirect(true);
        return FacturaRapidaPage;
    }