There are many ways you can do that, and the best way to do it depends on what you are looking to achieve on the rest of the game. So I will give you a few ideas, and you try to see if any of them works with what you are trying to do, and if they don't you come back here and try to be a bit more specific on what's going on on that game of yours.
First thing I would try is a "system > for each" and make the slot spawn the object. This will select each slot once and spawn the object for each one of them.
Another look would set at the slot properties a variable to define if the object is occupied or not. After that every time you want to spawn something you go "system>pick by comparison" and compare the variable of the slot with the value that tells if it is occupied or not.
And another way, that I wouldn't suggest that much, but it can still work is trying to check if the slot is overlapping the object you want it to spawn, and if it is not then spawn it.
Again, see if any of this helps and if it doesn't just come back here o/