I just searched for a Singleton implementation with AutoHotkey and I found the following: http://www.programering.com/a/MDN4YDMwATU.html
It's quite simple, so I thought I share my finding.
Basically spoken, a Singleton is a design pattern which ensures that a class has only one instance. Most common use for a Singleton (at least for me) is as a replacement for global variables. If you want to know more about Singletons have a look at Wikipedia(or google for it).
Here's the implementation suggestion for a Singleton with AutoHotkey - All credits go to: http://www.programering.com/a/MDN4YDMwATU.html