Duping items with /dupe willl check for stacked items
/gamerule allowStackedItemsDupe false gives you an item instead of multiplicating the item in main-hand
Duping with /dupefast and with allowStackedItemsDupe set to true will NOT check for stacked items
/gamerule allowStackedItemsDupe true allows you to dupe stackable items(swords, tools, armor, e.t.c)