Getting the instance number of a collided object

Okay, so what I'm trying to do is make it so if one instance of an object collides with another instance of the same object, it will repel the first one away from the second one. However, it's proving difficult to get the object to avoid repelling from itself. However, if I could get the instance of the object that it collides with, it would be much easier.

Thanks!

scirra.com/manual/124/system-conditions

Pick nth Instance is what you want. About halfway down the page.