Author: Jonas Bilinkevicius 

How can I prevent the user from running my program twice during the same Windows 
session? I want to force the user to log into Windows again before my application 
can be started a second time.

Answer:

A way to make a program only be able to run once in every session is to create a 
unique global atom string on first run. Then on the following run, check if the 
string exists, and not run the program if the string atom is present. For example:
1   
2   procedure TForm1.FormShow(Sender: TObject);
3   var
4     atom: Integer;
5   begin
6     if (GlobalFindAtom('This_is_some_unique_text') = 0) then
7       atom := GlobalAddAtom('This_is_some_unique_text')
8     else
9     begin
10      ShowMessage('This application can only be run once for every Windows Session.');
11      Close;
12    end;
13  end;