Now I noticed two of my chest both having the same item but in two chests, which are both not full:Storage chests - Things only end up here under two circumstances. First, bots will empty active provider chests into storage if there are no requesters that need those things. Second, construction bots will take stuff that they dismantle to storage chests. Note that bots will try to keep storage "segregated" - that is, they will prefer to put iron ore in a chest that already has iron ore in it first. Failing that, they will look for an empty storage chest. Only if none of those are available will they put the iron ore in a chest with something else.

At least one of them always had bot in them and for sure none of them got full with bots so no spill over.