I think containers.
Besides that. Lets gamble. You must have a condition somewhere that picks several map objects, and you follow up by picking the iso objects that matches with only the first instance of map object in the picklist. A 'for each object" will solve that. Or a 'on collision' in stead of 'is overlapping'.
Say you have several bullets somewhere in the air, or a dense pack enemy's.
Now the condition 'bullet is overlapping enemy' will result in a picklist containing many bullets and many enemy's.
map bullet is overlapping map enemy
____for each map bullet (sub event)
____pick iso bullet with the same instance variable as map bullet (same event)
..................................action ... destroy iso bullet
____for each map enemy (new sub event)
____pick iso enemy with the same instance variable as map enemy (same event)
..................................action ... destroy iso enemy
____an empty sub
..................................action ..destroy map bullet
..................................action ..destroy map enemy