Please help me with solving this error.
My trigger fires each time is record is inserted.It works fine with small data.Every night i get lot of inserts I am getting this error:: System.DmlException: Insert failed. first error: INVALID_FIELD_FOR_INSERT_UPDATE, cannot specify Id in an insert call: [Id]

below is my code
 

trigger Linktorecord on project__c (before insert,before update,after insert) {

    Map<String,product2> pMap=new Map<String,product2>();
    List<product2> p2List=new List<Product2>();
    List<Opportunity> oppList=New List<Opportunity>();
    List<Opportunity> oppList2=New List<Opportunity>();    
    oppList=[select id,name,accountid,serialnumber__c from opportunity where SerialNumber__c !=null];

    for(product2 pro  : [select id,productcode from product2]){
        pMap.put(pro.productcode,pro);
    }
   
  if(trigger.isinsert&&trigger.isbefore)
    for(Project__c I:trigger.new){

        if(i.Number__c != null && Pmap.Containskey(i.Number__c))
        {
            i.product__c=pMap.get(i.Number__c ).Id;
            break;
        } 
            
        else{
               
        Product2 newp =new Product2();
        if(i.number__c  !=null){        
        newp.Name=i.Number__c;
        }
        if(i.Number__c !=null){
        newp.ProductCode=i.Number__c;
        }
        p2List.add(newp);
        }
       
        insert p2List;
        if(p2List.size() >0){
        i.product__c=p2List[0].id; 
        }
       }
       
       if(trigger.isinsert && trigger.isafter){
       List<project__c> newproj =new List<project__c>();
       set<id> opids=new set<id>();
       newproj=trigger.new;
       Opplist2=[select id,SerialNumber__c,project__c from opportunity where Serialnumber__c =:newproj[0].Serialnumber__c];
       for(project__c I2:newproj){
         opids.add(i2.id);
       }
      if(!opids.isempty()){
      List<opportunity> oList =[select id,project__c from opportunity where Serialnumber__c =:newproj[0].Serialnumber__c];
       for(Opportunity ol:olist){
             
             ol.project__c =trigger.new[0].id;
       }      
      update olist;
      }       
    }
}