Issues with file type association

I am doing some tests on an application which is the default program for .testtxt file types (just a custom text format).
I have completed all the registry configurations (for the file association) and when i dblclick on a .testtxt file, it opens
with my app without problems.

But there are two issues about that:

1) When i try to open two different .testtxt files the same time, they execute my application two times when i want to execute
it just one and take arguments from command line using index 1 and 2 (0 carries my app startup path).

2) The same happens if my app is running and i try to open a .testtxt file using dlbclick. The file doesn't open the current running window,
but opens in a new one.

What could i do about these issues?

Go to the Application tab on your Project Properties and check the box for "Make single instance application".

what if my app is a wpf? :/