Do you think you could help me with another trigger I am having the same issues with?

This is the trigger that creates the Account Team member and Chatter Subscription upon insertion of the new Coverage Person record. it works fine via web but now testing via API (data loader or Excel Connector) am getting various errors.

Not all inserted Coverage Persons will relate to a User record.

If I insert a lot of ones that don't find a record, it seems to work.

If I insert ONE Coverage Person for a person that has a User account it works. But if I insert a list with more than the 1 record that matches a user, I get errors.

Here is my code:

trigger CPChatterandATMAdder on Coverage_Person__c(After Insert) {
    List < EntitySubscription > esSet = new List < EntitySubscription > ();
    List < Double > empIds = new List < Double > ();
    List < AccountTeamMember > atms = new List < AccountTeamMember > ();
    List < string > CpAcctId = new List < String > ();
    List < string > CPUsers = new List < string > ();
    List < AccountShare > ASSES = new List < AccountShare > ();
    for (Coverage_Person__c p: trigger.new) {
        empIds.add(p.Emp_Id__c);
        CPusers.add(p.User__c);
        CpAcctId.add(p.Account__c);
    }
    List < User > userList = [select id, Emp_Id__c from User where isActive = True and EMP_ID__c IN: empIds];
    List < EntitySubscription > presubsList = [select SubscriberID from EntitySubscription where ParentId IN: CpAcctId];
    //Set < EntitySubscription > presubs = new set < EntitySubscription > ();
    //presubs.addAll(presubsList);
     Set<ID> presubs = new set<ID>();
      for(Entitysubscription es:presubsList){presubs.add(es.subscriberID);} 
      
    for (Coverage_Person__c p: trigger.new) {
        for (User a: userList) {
            if (! preSubs.contains(a.id))
            //if (preSubs.contains(a) == false)
            //         if( preSubsList.isempty())
            {
                EntitySubscription follow = new EntitySubscription(ParentId = p.Account__c, subscriberid = a.id);
                esSet.add(follow);
                AccountShare ASS = new AccountShare(AccountID = p.Account__c, UserOrGroupId = a.id, AccountAccessLevel = 'Edit', OpportunityAccessLevel = 'None');
                ASSES.add(ASS);
            }
            if (p.Emp_Id__c == a.Emp_Id__c) {
                AccountTeamMember atm = new AccountTeamMember(AccountId = p.Account__c, UserId = a.id, TeamMemberRole = p.Coverage_Role__c);
                atms.add(atm);
            }
        }
    }
    insert atms;
    insert ASSES;
    insert esSet;
}

 here is my error:

Insert Row Failed:
CPChatterandATMAdder: execution of AfterInsert

caused by: System.DmlException: Insert failed. First exception on row 2; first error: DUPLICATE_VALUE, duplicate value found: <unknown> duplicates value on record with id: <unknown>: []

Trigger.CPChatterandATMAdder: line 41, column 1

thanks!