First you would have to pick one instance, and save its x, and y to a variable, then you would need to make it so that instance is not picked again. So..
On you condition/ trigger picking your object
- call function("myfunction") set parameter index 0 to object.x, set parameter index 1 to object.y
-- set object instance variable variable ("dontPickMe") to 1
-object compare variable("dontPickMe") =0
--object pick closest to object.variable((myfunction.parameter(0),myfunction.parameter(1))
So basically add a function object to get out of the picking issue.