Another example in R210. See video, 37:00. Also an example of this (minor) bug.
In this case, it looks like maybe the second item was not placed explicitly in the stockpile, but just dumped somewhere nearby after the person formerly carrying it needed rid of it (perhaps their job canceled for some reason). For this random dump-nearby behavior, I expect they don’t check if the spot they select is a stockpile and/or already has an item on it.