One instance with a twist

["Lucas van Dijk" <luckyluke56 gmail com>]

Hi, 

I'm creating a program, and I want the following feature, but I have no idea how to implent it:

When my program is running, and someone calls from the commandline this: 
> myprogram afile.txt
Usually, a new instance of the program would start, but I want to do something with the command line arguments, and use that argument in an existing instance. (Like adding a new Tab to a notebook, for file editting, instead of a new window)

Does anybody know how I can do this?

Thanks in advance,
-- 
Lucas