To me, "in which" seems sloppy at best. "which" by itself would be even less correct.
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MaengIn prisons where prisoners have to use buckets as toilets, slopping out is the practice in which they empty the buckets.I would rewrite this as follows: In prison, in cases where prisoners have to use buckets as toilets, the practice of emptying the buckets is called slopping-out.