The problem is your picking.
There are no way for C2 to know which skeleton you are referring to, so it just select all skeletons that match your picking.
And since you don't actually pick any in the event, but you use "Set skeleton Not Mirrored", it will just select all skeletons. The blue Keeperbox, is not really needed, you might as well just use the skeleton sprite it self. As you will now have twice as many units than needed.
Anyway the problem is not easily solved, as "bad" picking is something that affect you whole game. Meaning that the structure for picking the correct skeletons are not there, so to fix your problem you have to redesign how these skeletons and Keeperboxes are linked together, so you can pick them correctly.
So you need to really understand how picking works, otherwise you will keep running into weird problems. Would probably say that's the most important thing to understand in C2 when you start using it. Because if you get it wrong from the start when you create a game, there is a very big chance that you will have to spend a very long time solving problems with it later on.
This is from a former post I wrote for someone with similar picking problems, maybe that will help you as well.
It sounds like you (not uncommon) get a bit confused about how picking works in C2.
But the way you can look at picking of objects is like a sorting or reduction machine.
Imagine you have 100 workers with the following attributes.
100 Workers in total
20 Workers are wearing a red shirt.
40 Workers are wearing a blue shirt.
30 Workers are wearing a yellow shirt.
10 workers are not wearing a shirt.
30 Workers are between 40 and 50 years old.
50 Workers are between 20 and 30 years old,
20 Workers are between 60 and 80 years old (No pension for these poor bastards!)
Lets say you want all workers wearing blue shirts and that are above 60 year old to either go home or go to lunch.
So what happens is:
Pick all Workers : You pick all workers that match the condition that they are workers, in this case all are so its 100. (You don't really need this, its only for the explanation.)
Workers must have blue shirts : This will reduce the number to 40 workers.
Workers must be older than 60 : Depending on how many of those 40 workers matching this you will get a new number that further reduce or sort the amount of workers matching the former condition. Lets assume that its 15 workers.
Now we cant reduce the amount anymore and we have found all the workers that match all our conditions. And since we want them to either go home or to lunch, and don't care how many goes where. We can do the following.
For each Worker : This will go through each of the 15 remaining workers and tell each of them to either go home or to lunch.
Go home or Go to Lunch
So that's pretty much how it works whenever you pick objects, that you keep reducing or sorting them until those that remains fit whatever you want. And then if you want them to do different things you throw a "For each" at the end and then the options they have afterwards.