How to prevent multiple instances of external application

[shepherd]

How can I ensure that when I run an external application from an AHK script, that it only runs one instance?

Background: I have a program on my PC called eSignal. If I double click a file with .ach extension it will open eSignal. Under windows XP it would only open one instance no matter how many times I double clicked it. Unfortunately, under windows 7, it opens a new instance each time.

I would like to use AHK to reproduce the XP behaviour under windows 7 if possible.

Sorry if I've explained this badly but I'm fairly new to this. Any pointers would be appreciated.

[sinkfaze]

I don't have the ability to build a testable script for this at the moment, but here are a couple of good starting points:

How to detect a double-click ??
Explorer: get name of file under cursor

[shepherd]

Thanks sinkfaze.

To clarify, I want a script that will open multiple .ahk files under just one instance of eSignal. Currently I have a script that will open multiple .ahk files but every time a new instance of eSignal is opened, which is no good.

[Osprey]

Have you checked to make sure that it's not a setting in eSignal that you previously had enabled on your XP system but have neglected to re-enable since installing it on Windows 7? Some applications (like Notepad++) have a "single instance" setting so that you can toggle that behavior.

[shepherd]

Thanks Osprey. I looked for that but couldn't find anything. eSignal technical support knew nothing about it either.