I have controler extention in which I use a query to populate a picklist:

public class passengerExtention {

    
    private final Individuele_Boeking__c boeking;
    public string selectedArea {get; set;}

    public passengerExtention(ApexPages.StandardController controller) {

        this.boeking = (Individuele_Boeking__c) controller.getRecord();
    }
    
    //Picklist Area
    public list<selectoption> getAreaOptions(){
        
        
        //Verzamel waarden voor picklist
        SelectOption firstOption = new SelectOption('', ' - Choose Area - ');
        
        list<selectoption> Areas= new list<selectoption>();        
        list<Ships_location__c> selectedArea= [SELECT Area__c
                                               FROM Ships_location__c
                                               ORDER BY Area__c ];
                                               
        //Vul picklist                                       
        Areas.add(firstOption);
        
        for(Ships_location__c sl:selectedArea){
            Areas.add(new selectoption(sl.Area__c,sl.Area__c));
        }
        return Areas;     
    }

This works nicely, but it gives a lot of double values in the picklist...
So I tried to transform it with a GROUP BY query but I cannot get it work:

the result sofar is:

public class passengerExtention {

    
    private final Individuele_Boeking__c boeking;
    public string selectedArea {get; set;}

    public passengerExtention(ApexPages.StandardController controller) {

        this.boeking = (Individuele_Boeking__c) controller.getRecord();
    }
    
    //Picklist Area
    public list<selectoption> getAreaOptions(){
        
        
        //Verzamel waarden voor picklist
        SelectOption firstOption = new SelectOption('', ' - Choose Area - ');
        
        list<selectoption> Areas= new list<selectoption>();        
        list<AggregateResult> selectedArea = [SELECT Area__c, COUNT(id)
                                               FROM Ships_location__c
                                               GROUP BY Area__c
                                               ORDER BY Area__c ];
                                               
        //Vul picklist                                       
        Areas.add(firstOption);
        
        for(AggregateResult sl:selectedArea){
            Areas.add(new selectoption(sl.Area__c,sl.Area__c));
        }
        return Areas;     
    }
   
The error is invalid field Area__c for sObject AggregateResult...

I would appreciat any suggestion.

Thanks,

Robert