multi instance file opening problem

Apologies if this is the wrong group for this but I am trying to find
out the reason behind programmes such as Notepad being able to open
the same file in separate instances of the programme? On occasions
I've made the mistake of opening the same file twice which can be
really confusing and annoying.

Is there a way to prevent a file being opened more than once in
separate instances of the programme? I found ways to prevent multi
instances of an application being opened but that's not quite what i
want since then i can't open other files of the same type.

many thanks