When I open my world and go to a place where I placed Item frames , I see stacks of items . This is exactly 1 stack per every item frame and 1 stack per item which is put into teh frame . Item frame and the item in it is normally in it's proper place.
Items on the photo are outside and above them I have my item frames , which are placed on the wall, so I noticed , that these duplicated items pass through the first blocks they hit.
Having 20 item frames causes so heavy lags that minecraft crashed once