Getting dupicate agent names - salesforce

Question

Here am using map ,not abled to understand how to solve the duplicate name,i want one agent name against number of payments(child of opp)
how do I get unique names of users
public class First_Pay_ClearRates_Controller {
#AuraEnabled
public static Map<Id,User> fetchUsers() {
// List<User> userList= [SELECT Id, Name FROM User WHERE Profile.Name = 'Sales' LIMIT 20];
Map<Id,User> userMap= new Map<Id,User>([SELECT Id, Name FROM User WHERE Profile.Name = 'Sales' LIMIT 20]);
return userMap;
}
#AuraEnabled
public static List<Opportunity> getOpps(Map<Id,User> usrMap){
Map<Id,List<Opportunity>> oppMap = new Map<Id,List<Opportunity>>();
List<Opportunity> oppList = new List<Opportunity>();
List<Payments__c> payList= new List<Payments__c>();
List<Payments__c> payList1= new List<Payments__c>();
// prev key Agent_Name_User__c
// AND First_Payment_Date__c=THIS_MONTH
for(Opportunity o: [SELECT Id,First_Payment_Date__c,OwnerId,Owner.Name ,of_Payments_Made__c,PaymentsCounter__c,CollectedPaymentCount__c,First_Client_Payment_Collected_Date__c From Opportunity WHERE OwnerId IN: usrMap.keyset() AND First_Payment_Date__c!=null LIMIT 20]){
if(!oppMap.containsKey(o.OwnerId)){
oppMap.put(o.OwnerId,new List<Opportunity>());
}
o.PaymentsCounter__c=0;
o.CollectedPaymentCount__c=0;
//AND Due_Date__c=THIS_MONTH
for(Payments__c p :[Select Id,Opportunity__c,Payment_Collected__c,Due_Date__c FROM Payments__c WHERE Opportunity__c =: o.Id ]){
if(!p.Payment_Collected__c){
payList.add(p);
o.PaymentsCounter__c= payList.size();
}else if(p.Payment_Collected__c){
payList1.add(p);
o.CollectedPaymentCount__c= payList1.size();
}
}
oppList.add(o);
oppMap.put(o.OwnerId,oppList);
}
return oppList;
}