sharepoint online – Using CSOM can i ensure the user using the login user id instead of the login username

I have the following CSOM code inside my remote event receiver:

ListItem listItem = context2.Web.GetList(context2.Web.ServerRelativeUrl + "/lists/Assets").GetItemById(listItemID);
context2.Load(listItem);
context2.ExecuteQuery();
FieldUserValue creator = listItem("Author") as FieldUserValue;

listItem.BreakRoleInheritance(false, false);
listItem.RoleAssignments.Add(context2.Web.EnsureUser(creator.LookupValue), new RoleDefinitionBindingCollection(context2) { contributeDef });

The above code will get a list item >> assign the author of the item inside the listitem permission. The code will work on 99% of the cases.. but we face this case:-

1. there is an internal user named user.ABC
2. there is another external user named also user.ABC

So my above code will always select the internal username, even if the author was the external user.

So can I improve my code by passing the user id to the EnsureUser method? or another way to assign the Author of a list item to the list item permission?