How to ensure my pb app running only once?

[infratec]

You rely on the lock file mechanism of the filesystem.
If your home directory comes over a network connection you'll run into trouble.
And a second user (on a terminal server) can start the program also a second time.

[skywalk]

I use the same approach as infratec. Easy to allow option of 1 or multiple instances.

[kpeters58]

You rely on the lock file mechanism of the filesystem.
If your home directory comes over a network connection you'll run into trouble.
And a second user (on a terminal server) can start the program also a second time.

While technically true, my apps will never be run under such conditions, as they all are consumer apps - nothing that would ever be run in high/higher end corporate environments.

And to your second point: Just don't use the user's profile directory, but something public - problem solved