[SalesForce] Compare a string set and a list with Contains

I am creating two lists for a multi-select picklist. The available selections are being created through a query. During an edit I would like to only have the values which have not yet been selected show up as available options. So, I want to compare the results of the query to the values of the field and only keep the values not in the field.

I am attempting to do this with Contains, but I'm not getting results. I'm not sure if this is a viable approach to get what I am attempting.

Here is where I'm at right now. Essentially, I'm running two queries : One to get all of the possible options (options) and one to get the values of the field Subsidiaries_On_Contract in the record I am currently editing. (I'm not sure if there's an easier way to get that value). The name of that set is !selectedItems.

Here is the code. As it is now I'm still getting all of the possible choices as Available by referring to !items, instead of only getting those which are not in the Subsidiaries_And_Brands__c field.

public class MycontrollerSFDCBETA2b
{
private final Contract_Overview__c contract;
public String rightOptionsHidden { get; set; }
public String leftOptionsHidden { get; set; }
public String selectedMulPickKeyTech{get;set;} 
public string names{get;set;}
public string accountid{get;set;}
public String message { get; set; }
public List<SelectOption> options { get; set; }
public List<SelectOption> selectedSubs2 { get; set; }

    public MycontrollerSFDCBETA2b(apexpages.standardcontroller controller)
        {
        this.contract = (Contract_Overview__c) controller.getRecord();
        }

    public pageReference  execute()
        {
        return null;
        }

    public list<selectoption> getitems()
        {
        accountid=contract.Account__c;

        Set<String> selectedItems = new Set<String>();


        for(Contract_Overview__c x : [Select Id, Subsidiaries_On_Contract__c from Contract_Overview__c where id =:accountid])
              selectedItems.add(x.Subsidiaries_On_Contract__c); //aggregate all selected values

        selectedMulPickKeyTech=contract.Subsidiaries_On_Contract__c;
        selectedMulPickKeyTech=selectedMulPickKeyTech.replace('[','');                
        selectedMulPickKeyTech=selectedMulPickKeyTech.replace(']','');

        String[] selectedvalues = selectedMulPickKeyTech.split(',');
        selectedSubs2 = new list<SelectOption>();
        for (String selectedvalue: selectedvalues) 
            {
            selectedSubs2.add(new SelectOption(selectedvalue,selectedvalue));
            }

        List<selectoption> options= new list<selectoption>();
            if(accountid != null)
            {
            account a =[select name , (select name from Subsidiaries_and_Brands__r) from account where id =:accountid];
                for(SubsidiariesAndBrands__c s : a.Subsidiaries_and_Brands__r)
                    {
                        //check that selected items dont already contain the keys
                        if(!selectedItems.contains(s.name))
                    options.add(new SelectOption(s.name,s.name));
                    }
            }
            else
            options.add(new SelectOption('None','None'));
            return options;
        }

    public void save()
        {
        message = null ;       
        Boolean first = true;
           for ( SelectOption so : selectedSubs2 ) 
            {
                if (first) 
                    {
                    message = message ;
                    }
                    message = message + ', ' + so.getValue() ;
                    first = false;
            }        
                message = message.removeStart('null, ');
                message = message.normalizeSpace();
                message = '['+message+']';
                contract.Subsidiaries_On_Contract__c=message;

        update contract;

        }
}

Thank you very much for any help you can give. I really appreciate it.