Duplicate Value in field updateed by trigger.
My objective is to have the names of the campaign member in a field called Acteurs in Campaign separate by comma. 
 The code belows works, but i have the following issue:
   - All the campaignMember Name, are not displays in the campaign Custom field(Acteurs__c)
   - when i update the campaign members, it re-copy the name of all to the campaign member  to the Custom field(Acteurs__c) which cause duplicate name.
     
     Below are my Code:

Trigger allActors on CampaignMember (after update, after insert, after delete, after undelete){
    List <CampaignMember> cm = (Trigger.isInsert|| Trigger.isUnDelete) ? Trigger.new : Trigger.old;
    
    List <Id> campId = new List<Id> ();
    
    for (CampaignMember AllMember : cm) {
           campId.add(AllMember.CampaignId);
   }
   
   List< Campaign> campaignList = [ SELECT id, Acteurs__c, (SELECT Name  from  CampaignMembers) From Campaign 
                                    Where id in:campId];
                                    
    for (Campaign camp: campaignList){
        if (camp.CampaignMembers.size()>0){
            //camp.Name = string.valueOf(camp.CampaignMembers[0].Name);
            for(integer i =1; i < camp.CampaignMembers.size(); i++)
                camp.Acteurs__c = camp.Acteurs__c+';'+string.valueOf(camp.CampaignMembers[i].Name);
        }
            else
            camp.Acteurs__c = null;
        }   
                    update campaignList;

    }
	
	
	//Test Class
	
	@isTest Private class AllActorsTest{
   
    @isTest static void InsetCm(){
    Campaign c = new Campaign ();
    c.Name = 'My Campaign';
    Insert c;
        
    CampaignMember cm = new CampaignMember();
    contact cont = new Contact (firstName='Bijou', lastName ='Girbson');
    Upsert cont;  
    cm.CampaignId = c.Id;
    cm.ContactId = cont.Id;


    insert cm;
        
 
    Delete cm;

   }
}