ShellExecute - only one instance

Hi,
I want to execute/view a program with shellexecute, but when there's already an instance of this program this instance should be activated (as in, restore/focus regain/etc).

The program im building is called 'open.exe'

When you run 'open test.txt', a shellexecute 'test.txt' is done, wich results in notepad beeing executed with test.txt as parameter (?).

when you run 'open test.txt' a second time, a new notepad instance will be created with test.txt as param.

I want to skip this new instance the second time , but only activate the first instance.

Ps. when a instance of 'test.txt - Notepad' is opened, a call like 'open test2.txt' should result in a new notepad instance with test2.txt as param.

I'm wrestling with this problem for about 2 weeks now, and i'm getting very depressed... Please help.