Design and Architecture

1. How to handle multiple versions of the same file being dropped into the watch folder on different PC's at the same time. With latency, I guess I could always take the last file, but it's still a concern.
2. How to deal with a file or folder being deleted or edited during a sync. Example, user creates a file/folder, the the sync starts, then as it's syncing, deletes it? or opens it with some app?

I am trying to design a class for hierarchical data, I have some categories and dynamic number of subcategories for ex:
1.Technology --> C# --> Threading
                 -->IO
                 -->Exception
2.Domain --> Finance-->Equity
                  -->IR
                  -->Currency

like that multiple categories user can configure and then he will be able to add articles for any of above category. I have multiple thoughts like below codes and having confusion between below designs of classes 

    Public class Category{
        public int Id { get; set;}
        public string description { get; set;}
        public int ParentCategoryId {get; set;}
    }
    
    Public class Category{
        public int Id { get; set;}
        public string description { get; set;}
        public List<Category>SubCategories {get; set;}
    }
    
    Public class Category{
        public int Id { get; set;}
        public string description { get; set;}
        public Category Parent {get; set;}
    }