Data is not be duplicated to the two slots. Imagine otherwise, when slot A would be active and you install an app or set something, later when you switch to slot B, that would be 'lost',' you would see something else you previously installed/set maybe a month or two ago, when slot B was last time active
Only system partitions are duplicated to the slots A and B. Data partition (with user apps and settings) and Internal memory (with your documents, photos, etc) are not duplicated. There is only one/unique Data partition, no matter if currently slot A or B is active. When you Format Data from TWRP, it formats that one and unique Data partition