I am writing this trigger to prevent duplicate entry. But this trigger runs even at time of creating new record..How to resolve?

trigger TriggerOpp on Opportunity (before insert,before update) {
   set<id> setid=new set<id>();
    for(Opportunity opp:trigger.new)
    {
       setid.add(opp.AccountId);
    }
    
    Map<id,Account> acmap=new map<id,Account>();
    
    for(account acc:[select id,Name,(select id,Name,StageName from Opportunities) from Account where id in:setid])
    {
        acmap.put(acc.id,acc);
    }
    
    for(Opportunity opp:trigger.new)
    {
        if(acmap.containsKey(opp.AccountId) && acmap.get(opp.AccountId).Opportunities!=null)
        {
        
           {
               if(opp.StageName=='Prospecting')
               {
                     (opp.AccountId).addError('Duplicate value');
               }
           }
        }
    }
}

I have junction object "Author-Research Paper". This object has two master detail relationship with objects "Authors" and "Research Paper".
Now,there must not be more than 5 Authors for single research paper. How to write validtion rule for it?